diff --git a/crates/citygen/src/lib.rs b/crates/citygen/src/lib.rs
index b68ced8..8d3ed14 100644
--- a/crates/citygen/src/lib.rs
+++ b/crates/citygen/src/lib.rs
@@ -2,6 +2,14 @@
 
 pub mod geom;
 pub mod ids;
+pub mod net;
+pub mod params;
+pub mod tile;
+pub mod world;
 
 pub use geom::{Dir, Grid, Pos, Rect, Size};
 pub use ids::*;
+pub use net::*;
+pub use params::*;
+pub use tile::*;
+pub use world::*;
diff --git a/crates/citygen/src/net.rs b/crates/citygen/src/net.rs
new file mode 100644
index 0000000..e93f817
--- /dev/null
+++ b/crates/citygen/src/net.rs
@@ -0,0 +1,89 @@
+//! The Net: subnets, nodes and links anchored to physical tiles.
+
+use crate::geom::Pos;
+use crate::ids::{BuildingId, DistrictId, NodeId, SubnetId};
+use crate::world::StratumKind;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, Default, PartialEq, Serialize, Deserialize)]
+pub struct Net {
+    pub subnets: Vec<Subnet>,
+    pub nodes: Vec<Node>,
+    pub links: Vec<Link>,
+}
+
+/// Undirected link between two nodes.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub struct Link(pub NodeId, pub NodeId);
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum SubnetOwner {
+    Building(BuildingId),
+    District(DistrictId),
+    City,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Subnet {
+    pub id: SubnetId,
+    pub owner: SubnetOwner,
+    pub security: u8,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum NodeKind {
+    Terminal,
+    Camera,
+    DoorLock,
+    LiftCtrl,
+    AdBoard,
+    LampGrid,
+    Server,
+    DataVault,
+    Ice,
+    Backbone,
+}
+
+impl NodeKind {
+    pub fn label(self) -> &'static str {
+        match self {
+            NodeKind::Terminal => "terminal",
+            NodeKind::Camera => "camera",
+            NodeKind::DoorLock => "door lock",
+            NodeKind::LiftCtrl => "lift control",
+            NodeKind::AdBoard => "ad board",
+            NodeKind::LampGrid => "lamp grid",
+            NodeKind::Server => "server",
+            NodeKind::DataVault => "data vault",
+            NodeKind::Ice => "ICE",
+            NodeKind::Backbone => "backbone",
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Node {
+    pub id: NodeId,
+    pub kind: NodeKind,
+    pub anchor: (StratumKind, Pos),
+    pub subnet: SubnetId,
+    pub building: Option<BuildingId>,
+}
+
+impl Net {
+    pub fn nodes_on(&self, k: StratumKind) -> impl Iterator<Item = &Node> {
+        self.nodes.iter().filter(move |n| n.anchor.0 == k)
+    }
+
+    pub fn node_at(&self, k: StratumKind, p: Pos) -> Option<&Node> {
+        self.nodes.iter().find(|n| n.anchor == (k, p))
+    }
+
+    pub fn subnet(&self, id: SubnetId) -> &Subnet {
+        &self.subnets[id.index()]
+    }
+
+    pub fn node(&self, id: NodeId) -> &Node {
+        &self.nodes[id.index()]
+    }
+}
diff --git a/crates/citygen/src/params.rs b/crates/citygen/src/params.rs
new file mode 100644
index 0000000..680ff9c
--- /dev/null
+++ b/crates/citygen/src/params.rs
@@ -0,0 +1,76 @@
+//! Generation parameters.
+
+use crate::geom::Size;
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
+pub enum SizePreset {
+    Small,
+    Medium,
+    Large,
+    Custom { w: u32, h: u32 },
+}
+
+impl SizePreset {
+    pub fn size(self) -> Size {
+        match self {
+            SizePreset::Small => Size { w: 160, h: 80 },
+            SizePreset::Medium => Size { w: 240, h: 120 },
+            SizePreset::Large => Size { w: 320, h: 160 },
+            SizePreset::Custom { w, h } => Size { w, h },
+        }
+    }
+
+    pub fn label(self) -> &'static str {
+        match self {
+            SizePreset::Small => "small",
+            SizePreset::Medium => "medium",
+            SizePreset::Large => "large",
+            SizePreset::Custom { .. } => "custom",
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub enum WaterMode {
+    None,
+    Bay,
+    River,
+}
+
+impl WaterMode {
+    pub fn label(self) -> &'static str {
+        match self {
+            WaterMode::None => "none",
+            WaterMode::Bay => "bay",
+            WaterMode::River => "river",
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct GenParams {
+    pub seed: u64,
+    pub size: SizePreset,
+    pub districts: u8,
+    pub density: f32,
+    pub wealth_skew: f32,
+    pub decay: f32,
+    pub water: WaterMode,
+    pub flavor: String,
+}
+
+impl Default for GenParams {
+    fn default() -> Self {
+        GenParams {
+            seed: 0,
+            size: SizePreset::Medium,
+            districts: 7,
+            density: 0.6,
+            wealth_skew: 0.0,
+            decay: 0.4,
+            water: WaterMode::Bay,
+            flavor: "default".into(),
+        }
+    }
+}
diff --git a/crates/citygen/src/tile.rs b/crates/citygen/src/tile.rs
new file mode 100644
index 0000000..9cc81b9
--- /dev/null
+++ b/crates/citygen/src/tile.rs
@@ -0,0 +1,216 @@
+//! Tiles: terrain, features and derived walkability.
+
+use crate::ids::{BuildingId, DistrictId};
+use serde::{Deserialize, Serialize};
+
+/// Base terrain of a tile. Every tile has exactly one.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum Terrain {
+    Void,
+    Water,
+    Sewage,
+    Road,
+    Sidewalk,
+    Alley,
+    Plaza,
+    Floor,
+    Wall,
+    Door,
+    Window,
+    Rooftop,
+    Walkway,
+    Bridge,
+    Tunnel,
+    Rail,
+    Stair,
+    Lift,
+    Ladder,
+    Grate,
+}
+
+pub const ALL_TERRAIN: [Terrain; 20] = [
+    Terrain::Void,
+    Terrain::Water,
+    Terrain::Sewage,
+    Terrain::Road,
+    Terrain::Sidewalk,
+    Terrain::Alley,
+    Terrain::Plaza,
+    Terrain::Floor,
+    Terrain::Wall,
+    Terrain::Door,
+    Terrain::Window,
+    Terrain::Rooftop,
+    Terrain::Walkway,
+    Terrain::Bridge,
+    Terrain::Tunnel,
+    Terrain::Rail,
+    Terrain::Stair,
+    Terrain::Lift,
+    Terrain::Ladder,
+    Terrain::Grate,
+];
+
+/// Named neon colours; the renderer maps them to RGB.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum Hue {
+    Cyan,
+    Magenta,
+    Pink,
+    Orange,
+    Yellow,
+    Green,
+    Blue,
+    Red,
+    White,
+}
+
+/// Optional dressing on top of the terrain.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum Feature {
+    Neon(Hue),
+    HoloAd,
+    Lamp,
+    Vent,
+    Puddle,
+    Trash,
+    Barrier,
+    Stall,
+    Sign(char),
+    Terminal,
+    Camera,
+    AcUnit,
+    Helipad,
+    Pipe,
+    Cable,
+}
+
+pub const ALL_FEATURES: [Feature; 15] = [
+    Feature::Neon(Hue::Cyan),
+    Feature::HoloAd,
+    Feature::Lamp,
+    Feature::Vent,
+    Feature::Puddle,
+    Feature::Trash,
+    Feature::Barrier,
+    Feature::Stall,
+    Feature::Sign('ｱ'),
+    Feature::Terminal,
+    Feature::Camera,
+    Feature::AcUnit,
+    Feature::Helipad,
+    Feature::Pipe,
+    Feature::Cable,
+];
+
+/// One cell of a stratum.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
+pub struct Tile {
+    pub terrain: Terrain,
+    pub feature: Option<Feature>,
+    pub district: Option<DistrictId>,
+    pub building: Option<BuildingId>,
+    /// Deterministic per-tile hash used for glyph variation and animation phase.
+    pub variant: u8,
+}
+
+impl Tile {
+    pub const VOID: Tile = Tile {
+        terrain: Terrain::Void,
+        feature: None,
+        district: None,
+        building: None,
+        variant: 0,
+    };
+
+    pub fn new(terrain: Terrain) -> Self {
+        Tile {
+            terrain,
+            ..Tile::VOID
+        }
+    }
+
+    pub fn is_walkable(&self) -> bool {
+        use Terrain::*;
+        let terrain_ok = matches!(
+            self.terrain,
+            Road | Sidewalk
+                | Alley
+                | Plaza
+                | Floor
+                | Door
+                | Rooftop
+                | Walkway
+                | Bridge
+                | Tunnel
+                | Rail
+                | Stair
+                | Lift
+                | Ladder
+                | Grate
+        );
+        let feature_ok = !matches!(
+            self.feature,
+            Some(
+                Feature::Barrier
+                    | Feature::Stall
+                    | Feature::AcUnit
+                    | Feature::Pipe
+                    | Feature::Terminal
+            )
+        );
+        terrain_ok && feature_ok
+    }
+
+    pub fn blocks_sight(&self) -> bool {
+        matches!(self.terrain, Terrain::Wall | Terrain::Door)
+    }
+
+    /// Outdoor ground at street level: roads, sidewalks, alleys, plazas.
+    pub fn is_outdoor_ground(&self) -> bool {
+        matches!(
+            self.terrain,
+            Terrain::Road | Terrain::Sidewalk | Terrain::Alley | Terrain::Plaza
+        )
+    }
+
+    pub fn is_building_part(&self) -> bool {
+        matches!(
+            self.terrain,
+            Terrain::Wall | Terrain::Floor | Terrain::Door | Terrain::Window
+        )
+    }
+
+    /// A vertical connector tile.
+    pub fn is_connector(&self) -> bool {
+        matches!(
+            self.terrain,
+            Terrain::Stair | Terrain::Lift | Terrain::Ladder
+        )
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn walkability_rules() {
+        assert!(Tile::new(Terrain::Sidewalk).is_walkable());
+        assert!(!Tile::new(Terrain::Wall).is_walkable());
+        assert!(!Tile::new(Terrain::Water).is_walkable());
+        let mut t = Tile::new(Terrain::Road);
+        t.feature = Some(Feature::Barrier);
+        assert!(!t.is_walkable());
+        t.feature = Some(Feature::Lamp);
+        assert!(t.is_walkable());
+        assert!(Tile::new(Terrain::Wall).blocks_sight());
+        assert!(Tile::new(Terrain::Door).blocks_sight());
+        assert!(!Tile::new(Terrain::Window).blocks_sight());
+        assert!(!Tile::new(Terrain::Window).is_walkable());
+        assert!(Tile::new(Terrain::Plaza).is_outdoor_ground());
+        assert!(Tile::new(Terrain::Door).is_building_part());
+        assert_eq!(ALL_TERRAIN.len(), 20);
+        assert_eq!(ALL_FEATURES.len(), 15);
+    }
+}
diff --git a/crates/citygen/src/world.rs b/crates/citygen/src/world.rs
new file mode 100644
index 0000000..cbc77c0
--- /dev/null
+++ b/crates/citygen/src/world.rs
@@ -0,0 +1,439 @@
+//! The city: strata, districts, buildings, connectors.
+
+use crate::geom::{Grid, Pos, Rect, Size};
+use crate::ids::{BuildingId, DistrictId};
+use crate::net::Net;
+use crate::params::GenParams;
+use crate::tile::Tile;
+use serde::{Deserialize, Serialize};
+
+/// A vertical level of the city. `Net` is reserved for the cyberspace projection.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, Serialize, Deserialize)]
+pub enum StratumKind {
+    Undercity,
+    Street,
+    Skyways,
+    Heights,
+    Net,
+}
+
+impl StratumKind {
+    pub const PHYSICAL: [StratumKind; 4] = [
+        StratumKind::Undercity,
+        StratumKind::Street,
+        StratumKind::Skyways,
+        StratumKind::Heights,
+    ];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            StratumKind::Undercity => "Undercity",
+            StratumKind::Street => "Street",
+            StratumKind::Skyways => "Skyways",
+            StratumKind::Heights => "Heights",
+            StratumKind::Net => "Net",
+        }
+    }
+
+    /// Position in the physical stack, bottom first.
+    pub fn index(self) -> usize {
+        match self {
+            StratumKind::Undercity => 0,
+            StratumKind::Street => 1,
+            StratumKind::Skyways => 2,
+            StratumKind::Heights => 3,
+            StratumKind::Net => 4,
+        }
+    }
+
+    pub fn from_index(i: usize) -> Option<StratumKind> {
+        match i {
+            0 => Some(StratumKind::Undercity),
+            1 => Some(StratumKind::Street),
+            2 => Some(StratumKind::Skyways),
+            3 => Some(StratumKind::Heights),
+            4 => Some(StratumKind::Net),
+            _ => None,
+        }
+    }
+
+    pub fn above(self) -> Option<StratumKind> {
+        match self {
+            StratumKind::Undercity => Some(StratumKind::Street),
+            StratumKind::Street => Some(StratumKind::Skyways),
+            StratumKind::Skyways => Some(StratumKind::Heights),
+            _ => None,
+        }
+    }
+
+    pub fn below(self) -> Option<StratumKind> {
+        match self {
+            StratumKind::Street => Some(StratumKind::Undercity),
+            StratumKind::Skyways => Some(StratumKind::Street),
+            StratumKind::Heights => Some(StratumKind::Skyways),
+            _ => None,
+        }
+    }
+}
+
+/// One full tile map plus its precomputed light.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Stratum {
+    pub kind: StratumKind,
+    pub tiles: Grid<Tile>,
+    /// 0 dark .. 255 bright.
+    pub light: Grid<u8>,
+}
+
+impl Stratum {
+    pub fn new(kind: StratumKind, size: Size) -> Self {
+        Stratum {
+            kind,
+            tiles: Grid::filled(size, Tile::VOID),
+            light: Grid::filled(size, 0),
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum Theme {
+    CorpCore,
+    NightMarket,
+    Megablocks,
+    CombatZone,
+    Docks,
+    Industrial,
+    NeonStrip,
+    OldTown,
+}
+
+impl Theme {
+    pub const ALL: [Theme; 8] = [
+        Theme::CorpCore,
+        Theme::NightMarket,
+        Theme::Megablocks,
+        Theme::CombatZone,
+        Theme::Docks,
+        Theme::Industrial,
+        Theme::NeonStrip,
+        Theme::OldTown,
+    ];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            Theme::CorpCore => "Corp Core",
+            Theme::NightMarket => "Night Market",
+            Theme::Megablocks => "Megablocks",
+            Theme::CombatZone => "Combat Zone",
+            Theme::Docks => "Docks",
+            Theme::Industrial => "Industrial",
+            Theme::NeonStrip => "Neon Strip",
+            Theme::OldTown => "Old Town",
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct District {
+    pub id: DistrictId,
+    pub name: String,
+    pub theme: Theme,
+    pub center: Pos,
+    pub wealth: f32,
+    pub decay: f32,
+    pub bounds: Rect,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum BuildingKind {
+    CorpTower,
+    Office,
+    Megablock,
+    Apartment,
+    Hotel,
+    Shop,
+    NoodleBar,
+    Bar,
+    Club,
+    Arcade,
+    Pawn,
+    Clinic,
+    Temple,
+    Warehouse,
+    Factory,
+    Depot,
+    Garage,
+    Parking,
+    Market,
+}
+
+impl BuildingKind {
+    pub fn label(self) -> &'static str {
+        match self {
+            BuildingKind::CorpTower => "corp tower",
+            BuildingKind::Office => "office",
+            BuildingKind::Megablock => "megablock",
+            BuildingKind::Apartment => "apartment",
+            BuildingKind::Hotel => "hotel",
+            BuildingKind::Shop => "shop",
+            BuildingKind::NoodleBar => "noodle bar",
+            BuildingKind::Bar => "bar",
+            BuildingKind::Club => "club",
+            BuildingKind::Arcade => "arcade",
+            BuildingKind::Pawn => "pawn shop",
+            BuildingKind::Clinic => "clinic",
+            BuildingKind::Temple => "temple",
+            BuildingKind::Warehouse => "warehouse",
+            BuildingKind::Factory => "factory",
+            BuildingKind::Depot => "depot",
+            BuildingKind::Garage => "garage",
+            BuildingKind::Parking => "parking",
+            BuildingKind::Market => "market",
+        }
+    }
+
+    pub fn is_corp(self) -> bool {
+        matches!(
+            self,
+            BuildingKind::CorpTower | BuildingKind::Office | BuildingKind::Megablock
+        )
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum HeightClass {
+    Low,
+    Mid,
+    Tall,
+}
+
+/// Inclusive range of physical strata a building occupies.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub struct StrataSpan {
+    pub lo: StratumKind,
+    pub hi: StratumKind,
+}
+
+impl StrataSpan {
+    pub fn from_class(class: HeightClass, basement: bool) -> Self {
+        let hi = match class {
+            HeightClass::Low => StratumKind::Street,
+            HeightClass::Mid => StratumKind::Skyways,
+            HeightClass::Tall => StratumKind::Heights,
+        };
+        let lo = if basement {
+            StratumKind::Undercity
+        } else {
+            StratumKind::Street
+        };
+        StrataSpan { lo, hi }
+    }
+
+    pub fn contains(self, k: StratumKind) -> bool {
+        k.index() >= self.lo.index() && k.index() <= self.hi.index()
+    }
+
+    pub fn iter(self) -> impl Iterator<Item = StratumKind> {
+        (self.lo.index()..=self.hi.index()).filter_map(StratumKind::from_index)
+    }
+
+    pub fn len(self) -> usize {
+        self.hi.index() - self.lo.index() + 1
+    }
+
+    pub fn is_empty(self) -> bool {
+        false
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Building {
+    pub id: BuildingId,
+    pub kind: BuildingKind,
+    pub district: DistrictId,
+    /// Footprint at Street level; the same rect on every stratum of `span`.
+    pub footprint: Rect,
+    pub span: StrataSpan,
+    pub name: Option<String>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Serialize, Deserialize)]
+pub enum ConnectorKind {
+    Stair,
+    Lift,
+    Ladder,
+    Manhole,
+    Jack,
+}
+
+impl ConnectorKind {
+    pub fn label(self) -> &'static str {
+        match self {
+            ConnectorKind::Stair => "stairs",
+            ConnectorKind::Lift => "lift",
+            ConnectorKind::Ladder => "ladder",
+            ConnectorKind::Manhole => "manhole",
+            ConnectorKind::Jack => "jack",
+        }
+    }
+}
+
+/// A vertical link between strata at one position.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct Connector {
+    pub kind: ConnectorKind,
+    pub pos: Pos,
+    /// Served strata, ordered low to high.
+    pub serves: Vec<StratumKind>,
+    /// Reachable from the street without entering a building.
+    pub public: bool,
+}
+
+impl Connector {
+    /// The strata directly below and above `from` in `serves`, or `None` if not served.
+    pub fn serves_from(
+        &self,
+        from: StratumKind,
+    ) -> Option<(Option<StratumKind>, Option<StratumKind>)> {
+        let i = self.serves.iter().position(|k| *k == from)?;
+        let below = if i > 0 {
+            Some(self.serves[i - 1])
+        } else {
+            None
+        };
+        let above = self.serves.get(i + 1).copied();
+        Some((below, above))
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct City {
+    pub seed: u64,
+    pub params: GenParams,
+    pub name: String,
+    pub size: Size,
+    pub strata: Vec<Stratum>,
+    pub districts: Vec<District>,
+    pub buildings: Vec<Building>,
+    pub connectors: Vec<Connector>,
+    /// Centre paths of the arterial roads, in generation order.
+    pub arterials: Vec<Vec<Pos>>,
+    pub net: Net,
+}
+
+impl City {
+    /// Four physical strata of `Void`, nothing else.
+    pub fn empty(params: GenParams) -> City {
+        let size = params.size.size();
+        City {
+            seed: params.seed,
+            name: String::new(),
+            size,
+            strata: StratumKind::PHYSICAL
+                .iter()
+                .map(|k| Stratum::new(*k, size))
+                .collect(),
+            districts: Vec::new(),
+            buildings: Vec::new(),
+            connectors: Vec::new(),
+            arterials: Vec::new(),
+            net: Net::default(),
+            params,
+        }
+    }
+
+    pub fn stratum(&self, k: StratumKind) -> Option<&Stratum> {
+        self.strata.iter().find(|s| s.kind == k)
+    }
+
+    pub fn stratum_mut(&mut self, k: StratumKind) -> Option<&mut Stratum> {
+        self.strata.iter_mut().find(|s| s.kind == k)
+    }
+
+    pub fn tile(&self, k: StratumKind, p: Pos) -> Option<&Tile> {
+        self.stratum(k).and_then(|s| s.tiles.get(p))
+    }
+
+    pub fn tile_mut(&mut self, k: StratumKind, p: Pos) -> Option<&mut Tile> {
+        self.stratum_mut(k).and_then(|s| s.tiles.get_mut(p))
+    }
+
+    pub fn walkable(&self, k: StratumKind, p: Pos) -> bool {
+        self.tile(k, p).is_some_and(|t| t.is_walkable())
+    }
+
+    pub fn connectors_at(&self, p: Pos) -> impl Iterator<Item = &Connector> {
+        self.connectors.iter().filter(move |c| c.pos == p)
+    }
+
+    pub fn district(&self, id: DistrictId) -> &District {
+        &self.districts[id.index()]
+    }
+
+    pub fn building(&self, id: BuildingId) -> &Building {
+        &self.buildings[id.index()]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::params::{GenParams, SizePreset};
+    use crate::tile::Terrain;
+
+    #[test]
+    fn span_iter_and_class() {
+        let s = StrataSpan::from_class(HeightClass::Tall, true);
+        assert_eq!(s.iter().collect::<Vec<_>>(), StratumKind::PHYSICAL.to_vec());
+        let s = StrataSpan::from_class(HeightClass::Mid, false);
+        assert_eq!(
+            s.iter().collect::<Vec<_>>(),
+            vec![StratumKind::Street, StratumKind::Skyways]
+        );
+        assert!(s.contains(StratumKind::Skyways));
+        assert!(!s.contains(StratumKind::Heights));
+        assert_eq!(s.len(), 2);
+    }
+
+    #[test]
+    fn city_empty_has_four_void_strata() {
+        let c = City::empty(GenParams {
+            size: SizePreset::Custom { w: 10, h: 5 },
+            ..GenParams::default()
+        });
+        assert_eq!(c.strata.len(), 4);
+        assert_eq!(
+            c.stratum(StratumKind::Heights).unwrap().tiles.size(),
+            Size { w: 10, h: 5 }
+        );
+        assert_eq!(
+            c.tile(StratumKind::Street, Pos::new(3, 3)).unwrap().terrain,
+            Terrain::Void
+        );
+        assert!(c.tile(StratumKind::Street, Pos::new(10, 3)).is_none());
+        assert!(!c.walkable(StratumKind::Street, Pos::new(3, 3)));
+    }
+
+    #[test]
+    fn connector_neighbors() {
+        let c = Connector {
+            kind: ConnectorKind::Lift,
+            pos: Pos::new(0, 0),
+            serves: vec![
+                StratumKind::Street,
+                StratumKind::Skyways,
+                StratumKind::Heights,
+            ],
+            public: false,
+        };
+        assert_eq!(
+            c.serves_from(StratumKind::Skyways),
+            Some((Some(StratumKind::Street), Some(StratumKind::Heights)))
+        );
+        assert_eq!(
+            c.serves_from(StratumKind::Heights),
+            Some((Some(StratumKind::Skyways), None))
+        );
+        assert_eq!(c.serves_from(StratumKind::Undercity), None);
+    }
+}