diff --git a/src/main.rs b/src/main.rs
index 05bd033..829f514 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -142,23 +142,21 @@ impl State {
         }
 
         // Build new map
-        let worldmap;
+        let mut builder;
         let current_depth;
         let player_start;
         {
             let mut worldmap_resource = self.ecs.write_resource::<Map>();
             current_depth = worldmap_resource.depth;
             let mut rng = self.ecs.write_resource::<RandomNumberGenerator>();
-            let (newmap, start) = map_builders::build_random_map(&mut rng, current_depth + 1);
-            *worldmap_resource = newmap;
-            player_start = start;
-            worldmap = worldmap_resource.clone();
+            builder = map_builders::random_builder(current_depth + 1);
+            builder.build_map(&mut rng);
+            *worldmap_resource = builder.get_map();
+            player_start = builder.get_starting_pos();
         }
 
         // Spawn things in rooms
-        for room in worldmap.rooms.iter().skip(1) {
-            spawner::spawn_room(&mut self.ecs, room, current_depth + 1);
-        }
+        builder.spawn_entities(&mut self.ecs);
 
         // Place the player and update resources
         let mut player_position = self.ecs.write_resource::<Point>();
@@ -198,21 +196,19 @@ impl State {
         }
 
         // Build a new map and place the player
-        let worldmap;
+        let mut builder;
         let player_start;
         {
             let mut worldmap_resource = self.ecs.write_resource::<Map>();
             let mut rng = self.ecs.write_resource::<RandomNumberGenerator>();
-            let (newmap, start) = map_builders::build_random_map(&mut rng, 1);
-            *worldmap_resource = newmap;
-            player_start = start;
-            worldmap = worldmap_resource.clone();
+            builder = map_builders::random_builder(1);
+            builder.build_map(&mut rng);
+            *worldmap_resource = builder.get_map();
+            player_start = builder.get_starting_pos();
         }
 
         // Spawn bad guys
-        for room in worldmap.rooms.iter().skip(1) {
-            spawner::spawn_room(&mut self.ecs, room, 1);
-        }
+        builder.spawn_entities(&mut self.ecs);
 
         // Place the player and update resources
         let (player_x, player_y) = (player_start.x, player_start.y);
@@ -533,16 +529,17 @@ fn main() -> rltk::BError {
     // Create seed.
     let mut rng = rltk::RandomNumberGenerator::new();
     // Use seed to generate the map.
-    let (map, player_start) = map_builders::build_random_map(&mut rng, 1);
+    let mut builder = map_builders::random_builder(1);
+    builder.build_map(&mut rng);
+    let player_start = builder.get_starting_pos();
+    let map = builder.get_map();
     // Insert seed into the ECS.
     gs.ecs.insert(rng);
 
     let player_name = "wanderer".to_string();
     let player_entity = spawner::player(&mut gs.ecs, player_start.x, player_start.y, player_name);
 
-    for room in map.rooms.iter().skip(1) {
-        spawner::spawn_room(&mut gs.ecs, room, 1);
-    }
+    builder.spawn_entities(&mut gs.ecs);
 
     gs.ecs.insert(map);
     gs.ecs.insert(Point::new(player_start.x, player_start.y));
diff --git a/src/map.rs b/src/map.rs
index a0aec24..31d528a 100644
--- a/src/map.rs
+++ b/src/map.rs
@@ -1,4 +1,3 @@
-use super::Rect;
 use rltk::{Algorithm2D, BaseMap, Point, Rltk, RGB};
 use serde::{Deserialize, Serialize};
 use specs::prelude::*;
@@ -19,7 +18,6 @@ pub const MAPCOUNT: usize = MAPHEIGHT * MAPWIDTH;
 #[derive(Default, Serialize, Deserialize, Clone)]
 pub struct Map {
     pub tiles: Vec<TileType>,
-    pub rooms: Vec<Rect>,
     pub width: i32,
     pub height: i32,
     pub revealed_tiles: Vec<bool>,
@@ -46,7 +44,6 @@ impl Map {
     pub fn new(new_depth: i32) -> Map {
         Map {
             tiles: vec![TileType::Wall; MAPCOUNT],
-            rooms: Vec::new(),
             width: MAPWIDTH as i32,
             height: MAPHEIGHT as i32,
             revealed_tiles: vec![false; MAPCOUNT],
diff --git a/src/map_builders/mod.rs b/src/map_builders/mod.rs
index 5bf7e33..03653f2 100644
--- a/src/map_builders/mod.rs
+++ b/src/map_builders/mod.rs
@@ -6,10 +6,13 @@ use common::*;
 use rltk::RandomNumberGenerator;
 use specs::prelude::*;
 
-trait MapBuilder {
-    fn build(rng: &mut RandomNumberGenerator, new_depth: i32) -> (Map, Position);
+pub trait MapBuilder {
+    fn build_map(&mut self, rng: &mut RandomNumberGenerator);
+    fn spawn_entities(&mut self, ecs: &mut World);
+    fn get_map(&mut self) -> Map;
+    fn get_starting_pos(&mut self) -> Position;
 }
 
-pub fn build_random_map(rng: &mut RandomNumberGenerator, new_depth: i32) -> (Map, Position) {
-    SimpleMapBuilder::build(rng, new_depth)
+pub fn random_builder(new_depth: i32) -> Box<dyn MapBuilder> {
+    return Box::new(SimpleMapBuilder::new(new_depth));
 }
diff --git a/src/map_builders/simple_map.rs b/src/map_builders/simple_map.rs
index 76a9258..2151e52 100644
--- a/src/map_builders/simple_map.rs
+++ b/src/map_builders/simple_map.rs
@@ -5,73 +5,98 @@ use super::{
 use rltk::RandomNumberGenerator;
 use specs::prelude::*;
 
-pub struct SimpleMapBuilder {}
+pub struct SimpleMapBuilder {
+    map: Map,
+    starting_position: Position,
+    depth: i32,
+    rooms: Vec<Rect>,
+}
 
 impl MapBuilder for SimpleMapBuilder {
-    fn build(rng: &mut RandomNumberGenerator, new_depth: i32) -> (Map, Position) {
-        let mut map = Map::new(new_depth);
-        let player_pos = SimpleMapBuilder::rooms_and_corridors(rng, &mut map);
+    fn get_map(&mut self) -> Map {
+        return self.map.clone();
+    }
 
-        return (map, player_pos);
+    fn get_starting_pos(&mut self) -> Position {
+        return self.starting_position.clone();
+    }
+
+    fn build_map(&mut self, rng: &mut RandomNumberGenerator) {
+        return self.rooms_and_corridors(rng);
+    }
+
+    fn spawn_entities(&mut self, ecs: &mut World) {
+        for room in self.rooms.iter().skip(1) {
+            return spawner::spawn_room(ecs, room, self.depth);
+        }
     }
 }
 
 impl SimpleMapBuilder {
-    fn rooms_and_corridors(rng: &mut RandomNumberGenerator, map: &mut Map) -> Position {
+    pub fn new(new_depth: i32) -> SimpleMapBuilder {
+        SimpleMapBuilder {
+            map: Map::new(new_depth),
+            starting_position: Position { x: 0, y: 0 },
+            depth: new_depth,
+            rooms: Vec::new(),
+        }
+    }
+
+    fn rooms_and_corridors(&mut self, rng: &mut RandomNumberGenerator) {
         const MAX_ROOMS: i32 = 30;
         const MIN_SIZE: i32 = 6;
         const MAX_SIZE: i32 = 10;
         const MAX_OFFSET: u8 = 32;
 
-        for idx in 0..map.red_offset.len() {
+        for idx in 0..self.map.red_offset.len() {
             let roll = rng.roll_dice(1, MAX_OFFSET as i32);
-            map.red_offset[idx] = roll as u8;
+            self.map.red_offset[idx] = roll as u8;
         }
-        for idx in 0..map.green_offset.len() {
+        for idx in 0..self.map.green_offset.len() {
             let roll = rng.roll_dice(1, MAX_OFFSET as i32);
-            map.green_offset[idx] = roll as u8;
+            self.map.green_offset[idx] = roll as u8;
         }
-        for idx in 0..map.blue_offset.len() {
+        for idx in 0..self.map.blue_offset.len() {
             let roll = rng.roll_dice(1, MAX_OFFSET as i32);
-            map.blue_offset[idx] = roll as u8;
+            self.map.blue_offset[idx] = roll as u8;
         }
 
         for _i in 0..MAX_ROOMS {
             let w = rng.range(MIN_SIZE, MAX_SIZE);
             let h = rng.range(MIN_SIZE, MAX_SIZE);
-            let x = rng.roll_dice(1, map.width - w - 1) - 1;
-            let y = rng.roll_dice(1, map.height - h - 1) - 1;
+            let x = rng.roll_dice(1, self.map.width - w - 1) - 1;
+            let y = rng.roll_dice(1, self.map.height - h - 1) - 1;
             let new_room = Rect::new(x, y, w, h);
             let mut ok = true;
-            for other_room in map.rooms.iter() {
+            for other_room in self.rooms.iter() {
                 if new_room.intersect(other_room) {
                     ok = false
                 }
             }
             if ok {
-                apply_room_to_map(map, &new_room);
+                apply_room_to_map(&mut self.map, &new_room);
 
-                if !map.rooms.is_empty() {
+                if !self.rooms.is_empty() {
                     let (new_x, new_y) = new_room.centre();
-                    let (prev_x, prev_y) = map.rooms[map.rooms.len() - 1].centre();
+                    let (prev_x, prev_y) = self.rooms[self.rooms.len() - 1].centre();
                     if rng.range(0, 2) == 1 {
-                        apply_horizontal_tunnel(map, prev_x, new_x, prev_y);
-                        apply_vertical_tunnel(map, prev_y, new_y, new_x);
+                        apply_horizontal_tunnel(&mut self.map, prev_x, new_x, prev_y);
+                        apply_vertical_tunnel(&mut self.map, prev_y, new_y, new_x);
                     } else {
-                        apply_vertical_tunnel(map, prev_y, new_y, prev_x);
-                        apply_horizontal_tunnel(map, prev_x, new_x, new_y);
+                        apply_vertical_tunnel(&mut self.map, prev_y, new_y, prev_x);
+                        apply_horizontal_tunnel(&mut self.map, prev_x, new_x, new_y);
                     }
                 }
 
-                map.rooms.push(new_room);
+                self.rooms.push(new_room);
             }
         }
 
-        let stairs_position = map.rooms[map.rooms.len() - 1].centre();
-        let stairs_idx = map.xy_idx(stairs_position.0, stairs_position.1);
-        map.tiles[stairs_idx] = TileType::DownStair;
+        let stairs_position = self.rooms[self.rooms.len() - 1].centre();
+        let stairs_idx = self.map.xy_idx(stairs_position.0, stairs_position.1);
+        self.map.tiles[stairs_idx] = TileType::DownStair;
 
-        let start_pos = map.rooms[0].centre();
-        return Position { x: start_pos.0, y: start_pos.1 };
+        let start_pos = self.rooms[0].centre();
+        self.starting_position = Position { x: start_pos.0, y: start_pos.1 };
     }
 }