diff --git a/src/dungeon.lua b/src/dungeon.lua
new file mode 100644
index 0000000..1d598c3
--- /dev/null
+++ b/src/dungeon.lua
@@ -0,0 +1,60 @@
+-- Owns every floor. Each level's layout is derived from the world seed and its
+-- depth alone, through a private rng that never touches the world rng stream, so
+-- a floor generates identically no matter when it is first entered. Generated
+-- floors are cached, which is what makes revisiting one restore its exact state.
+
+local Rng = require("rng")
+local Builder = require("builder")
+local Level = require("level")
+
+local Dungeon = {}
+Dungeon.__index = Dungeon
+
+local SALT = 0x9E3779B97F4A7C15
+
+function Dungeon.new(worldSeed, options)
+  return setmetatable({
+    worldSeed = worldSeed,
+    options = options or {},
+    levels = {},
+  }, Dungeon)
+end
+
+function Dungeon:levelSeed(depth)
+  return math.tointeger(self.worldSeed ~ (depth * SALT)) or 0
+end
+
+function Dungeon:get(depth)
+  if not self.levels[depth] then
+    local rng = Rng.new(self:levelSeed(depth))
+    self.levels[depth] = Builder.build(depth, rng, self.options)
+  end
+  return self.levels[depth]
+end
+
+function Dungeon:isGenerated(depth)
+  return self.levels[depth] ~= nil
+end
+
+function Dungeon:toData()
+  local depths = {}
+  for depth in pairs(self.levels) do
+    depths[#depths + 1] = depth
+  end
+  table.sort(depths)
+  local stored = {}
+  for _, depth in ipairs(depths) do
+    stored[#stored + 1] = { depth = depth, level = self.levels[depth]:toData() }
+  end
+  return { worldSeed = self.worldSeed, options = self.options, levels = stored }
+end
+
+function Dungeon.fromData(snapshot)
+  local dungeon = Dungeon.new(snapshot.worldSeed, snapshot.options)
+  for _, entry in ipairs(snapshot.levels) do
+    dungeon.levels[entry.depth] = Level.fromData(entry.level)
+  end
+  return dungeon
+end
+
+return Dungeon