diff --git a/src/exit.rs b/src/exit.rs
index f7eb20c..c523f1a 100644
--- a/src/exit.rs
+++ b/src/exit.rs
@@ -54,7 +54,7 @@ impl ToString for Transition {
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Exit {
     /// destination
-    pub room_id: u64,
+    pub room_id: String,
     /// id
     pub position: Position,
     pub effect: Transition,
@@ -67,7 +67,7 @@ impl FromStr for Exit {
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         let mut parts = s.split_whitespace();
-        let room_id = from_base36(parts.next().unwrap());
+        let room_id = parts.next().unwrap().to_string();
         let position = Position::from_str(parts.next().unwrap());
 
         if position.is_err() {
@@ -92,7 +92,7 @@ impl fmt::Display for Exit {
         write!(
             f,
             "{} {}{}",
-            self.room_id.to_base36(),
+            self.room_id,
             self.position.to_string(),
             self.effect.to_string()
         )
@@ -110,7 +110,7 @@ mod test {
         assert_eq!(
             Exit::from_str("a 12,13").unwrap(),
             Exit {
-                room_id: 10,
+                room_id: "a".to_string(),
                 position: Position { x: 12, y: 13 },
                 effect: Transition::None
             }
@@ -122,7 +122,7 @@ mod test {
         assert_eq!(
             Exit::from_str("a 12,13 FX slide_u").unwrap(),
             Exit {
-                room_id: 10,
+                room_id: "a".to_string(),
                 position: Position { x: 12, y: 13 },
                 effect: Transition::SlideUp
             }
@@ -133,7 +133,7 @@ mod test {
     fn test_exit_to_string() {
         assert_eq!(
             Exit {
-                room_id: 8,
+                room_id: "8".to_string(),
                 position: Position { x: 5, y: 6 },
                 effect: Transition::None
             }.to_string(),
@@ -145,7 +145,7 @@ mod test {
     fn test_exit_to_string_with_fx() {
         assert_eq!(
             Exit {
-                room_id: 8,
+                room_id: "8".to_string(),
                 position: Position { x: 5, y: 6 },
                 effect: Transition::FadeToWhite
             }.to_string(),
diff --git a/src/game.rs b/src/game.rs
index 83f9ab7..60b2efd 100644
--- a/src/game.rs
+++ b/src/game.rs
@@ -273,7 +273,7 @@ impl ToString for Game {
         }
 
         for sprite in &self.sprites {
-            if is_string_numeric(sprite.id.to_base36()) {
+            if is_string_numeric(sprite.id.clone()) {
                 segments.push(sprite.to_string());
             }
         }
@@ -281,7 +281,7 @@ impl ToString for Game {
         segments.push(self.avatar.to_string().replace("SPR a", "SPR A"));
 
         for sprite in &self.sprites {
-            if !is_string_numeric(sprite.id.to_base36()) {
+            if !is_string_numeric(sprite.id.clone()) {
                 segments.push(sprite.to_string());
             }
         }
diff --git a/src/item.rs b/src/item.rs
index 91a032e..78a0592 100644
--- a/src/item.rs
+++ b/src/item.rs
@@ -1,9 +1,9 @@
-use crate::{from_base36, optional_data_line, AnimationFrames, Image, ToBase36};
+use crate::{optional_data_line, AnimationFrames, Image};
 use crate::image::animation_frames_from_string;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Item {
-    pub id: u64,
+    pub id: String,
     pub animation_frames: Vec<Image>,
     pub name: Option<String>,
     pub dialogue_id: Option<String>,
@@ -32,7 +32,7 @@ impl From<String> for Item {
     fn from(string: String) -> Item {
         let mut lines: Vec<&str> = string.lines().collect();
 
-        let id = from_base36(&lines[0].replace("ITM ", ""));
+        let id = lines[0].replace("ITM ", "");
         let mut name = None;
         let mut dialogue_id = None;
         let mut colour_id: Option<u64> = None;
@@ -71,7 +71,7 @@ impl ToString for Item {
     fn to_string(&self) -> String {
         format!(
             "ITM {}\n{}{}{}{}",
-            self.id.to_base36(),
+            self.id,
             self.animation_frames.to_string(),
             self.name_line(),
             self.dialogue_line(),
diff --git a/src/mock.rs b/src/mock.rs
index 6dbd0e0..1a5b8c2 100644
--- a/src/mock.rs
+++ b/src/mock.rs
@@ -84,7 +84,7 @@ pub mod image {
 #[inline]
 pub fn avatar() -> Sprite {
     Sprite {
-        id: 0,
+        id: "0".to_string(),
         animation_frames: vec![
             Image {
                 pixels: vec![
@@ -102,7 +102,7 @@ pub fn avatar() -> Sprite {
             },
         ],
         name: None,
-        room_id: Some(0),
+        room_id: Some("0".to_string()),
         position: Some(Position { x: 2, y: 5 }),
         colour_id: None,
         dialogue_id: None,
@@ -130,7 +130,7 @@ pub fn tile_default() -> Tile {
 #[inline]
 pub fn sprite() -> Sprite {
     Sprite {
-        id: 10,
+        id: "a".to_string(),
         name: Some("hatch".to_string()),
         animation_frames: vec![Image {
             pixels: vec![
@@ -140,7 +140,7 @@ pub fn sprite() -> Sprite {
             ],
         }],
         dialogue_id: Some("SPR_0".to_string()),
-        room_id: Some(4),
+        room_id: Some("4".to_string()),
         position: Some(Position { x: 9, y: 7 }),
         colour_id: None,
         items: vec![]
@@ -150,7 +150,7 @@ pub fn sprite() -> Sprite {
 #[inline]
 pub fn item() -> Item {
     Item {
-        id: 6,
+        id: "6".to_string(),
         animation_frames: vec![Image {
             pixels: vec![
                 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -167,8 +167,8 @@ pub fn item() -> Item {
 #[inline]
 pub fn room() -> Room {
     Room {
-        id: 10,
-        palette_id: Some(9),
+        id: "a".to_string(),
+        palette_id: Some("9".to_string()),
         name: Some("cellar 7".to_string()),
         tiles: vec![
             "0".to_string(),
@@ -453,7 +453,7 @@ pub fn room() -> Room {
         exits: vec![ExitInstance {
             position: Position { x: 3, y: 3 },
             exit: Exit {
-                room_id: 3,
+                room_id: "3".to_string(),
                 position: Position { x: 10, y: 6 },
                 effect: Transition::None,
             },
@@ -479,7 +479,7 @@ pub fn game_default() -> Game {
         custom_font: None,
         text_direction: TextDirection::LeftToRight,
         palettes: vec![Palette {
-            id: 0,
+            id: "0".to_string(),
             name: None,
             colours: vec![
                 Colour {
@@ -500,8 +500,8 @@ pub fn game_default() -> Game {
             ],
         }],
         rooms: vec![Room {
-            id: 0,
-            palette_id: Some(0),
+            id: "0".to_string(),
+            palette_id: Some("0".to_string()),
             name: None,
             tiles: vec![
                 "0".to_string(),
@@ -768,7 +768,7 @@ pub fn game_default() -> Game {
         }],
         tiles: vec![self::tile_default()],
         avatar: Sprite {
-            id: 10,
+            id: "A".to_string(),
             animation_frames: vec![Image {
                 pixels: vec![
                     0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0,
@@ -777,14 +777,14 @@ pub fn game_default() -> Game {
                 ],
             }],
             name: None,
-            room_id: Some(0),
+            room_id: Some("0".to_string()),
             position: Option::from(Position { x: 4, y: 4 }),
             colour_id: None,
             dialogue_id: None,
             items: vec![]
         },
         sprites: vec![Sprite {
-            id: 10,
+            id: "a".to_string(),
             name: None,
             animation_frames: vec![Image {
                 pixels: vec![
@@ -794,13 +794,13 @@ pub fn game_default() -> Game {
                 ],
             }],
             dialogue_id: Some("SPR_0".to_string()),
-            room_id: Some(0),
+            room_id: Some("0".to_string()),
             position: Some(Position { x: 8, y: 12 }),
             colour_id: None,
             items: vec![]
         }],
         items: vec![Item {
-            id: 0,
+            id: "0".to_string(),
             animation_frames: vec![Image {
                 pixels: vec![
                     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
diff --git a/src/palette.rs b/src/palette.rs
index 5fde93c..f776910 100644
--- a/src/palette.rs
+++ b/src/palette.rs
@@ -1,9 +1,8 @@
 use crate::colour::Colour;
-use crate::{from_base36, ToBase36};
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Palette {
-    pub id: u64,
+    pub id: String,
     pub name: Option<String>,
     pub colours: Vec<Colour>,
 }
@@ -13,7 +12,7 @@ impl From<String> for Palette {
     fn from(string: String) -> Palette {
         let lines: Vec<&str> = string.lines().collect();
 
-        let id = from_base36(&lines[0].replace("PAL ", ""));
+        let id = lines[0].replace("PAL ", "");
 
         let name = match lines[1].starts_with("NAME") {
             true => Some(lines[1].replace("NAME ", "").to_string()),
@@ -46,7 +45,7 @@ impl ToString for Palette {
         }
         colours.pop();
 
-        format!("PAL {}\n{}{}", self.id.to_base36(), name, colours)
+        format!("PAL {}\n{}{}", self.id, name, colours)
     }
 }
 
@@ -60,7 +59,7 @@ mod test {
         let output = Palette::from("PAL 1\nNAME lamplight\n45,45,59\n66,60,39\n140,94,1".to_string());
 
         let expected = Palette {
-            id: 1,
+            id: "1".to_string(),
             name: Some("lamplight".to_string()),
             colours: vec![
                 Colour {
@@ -89,7 +88,7 @@ mod test {
         let output = Palette::from("PAL 9\n45,45,59\n66,60,39\n140,94,1".to_string());
 
         let expected = Palette {
-            id: 9,
+            id: "9".to_string(),
             name: None,
             colours: vec![
                 Colour {
@@ -116,7 +115,7 @@ mod test {
     #[test]
     fn test_palette_to_string() {
         let output = Palette {
-            id: 16,
+            id: "g".to_string(),
             name: Some("moss".to_string()),
             colours: vec![
                 Colour {
diff --git a/src/room.rs b/src/room.rs
index 94a3a90..0f4733c 100644
--- a/src/room.rs
+++ b/src/room.rs
@@ -5,14 +5,17 @@ use std::str::FromStr;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Room {
-    pub id: u64,
-    pub palette_id: Option<u64>, // optional in very early versions
+    pub id: String,
+    /// palette ID was optional in very early versions
+    pub palette_id: Option<String>,
     pub name: Option<String>,
-    pub tiles: Vec<String>, // tile ids
+    /// tile IDs
+    pub tiles: Vec<String>,
     pub items: Vec<Instance>,
     pub exits: Vec<ExitInstance>,
     pub endings: Vec<Instance>,
-    pub walls: Vec<u64>, // old way of handling walls...
+    /// old method of handling walls - a comma-separated list of tile IDs
+    pub walls: Vec<String>,
 }
 
 impl Room {
@@ -24,8 +27,7 @@ impl Room {
     #[inline]
     fn wall_line(&self) -> String {
         if self.walls.len() > 0 {
-            let ids: Vec<String> = self.walls.iter().map(|&id| id.to_base36()).collect();
-            optional_data_line("WAL", Some(ids.join(",")))
+            optional_data_line("WAL", Some(self.walls.join(",")))
         } else {
             "".to_string()
         }
@@ -34,7 +36,7 @@ impl Room {
     #[inline]
     fn palette_line(&self) -> String {
         if self.palette_id.is_some() {
-            optional_data_line("PAL", Some(self.palette_id.unwrap().to_base36()))
+            optional_data_line("PAL", Some(self.palette_id.as_ref().unwrap()))
         } else {
             "".to_string()
         }
@@ -47,13 +49,13 @@ impl From<String> for Room {
         let string = string.replace("ROOM ", "");
         let string = string.replace("SET ", "");
         let mut lines: Vec<&str> = string.lines().collect();
-        let id = from_base36(&lines[0]);
+        let id = lines[0].to_string();
         let mut name = None;
         let mut palette_id = None;
         let mut items: Vec<Instance> = Vec::new();
         let mut exits: Vec<ExitInstance> = Vec::new();
         let mut endings: Vec<Instance> = Vec::new();
-        let mut walls: Vec<u64> = Vec::new();
+        let mut walls: Vec<String> = Vec::new();
 
         loop {
             let last_line = lines.pop().unwrap();
@@ -61,11 +63,11 @@ impl From<String> for Room {
             if last_line.starts_with("WAL") {
                 let last_line = last_line.replace("WAL ", "");
                 let ids: Vec<&str> = last_line.split(",").collect();
-                walls = ids.iter().map(|&id| from_base36(id)).collect();
+                walls = ids.iter().map(|&id| id.to_string()).collect();
             } else if last_line.starts_with("NAME") {
                 name = Some(last_line.replace("NAME ", "").to_string());
             } else if last_line.starts_with("PAL") {
-                palette_id = Some(from_base36(&last_line.replace("PAL ", "")));
+                palette_id = Some(last_line.replace("PAL ", ""));
             } else if last_line.starts_with("ITM") {
                 let last_line = last_line.replace("ITM ", "");
                 let item_position: Vec<&str> = last_line.split(' ').collect();
@@ -211,7 +213,7 @@ impl Room {
         format!(
             "{} {}\n{}{}{}{}{}{}{}",
             room_type.to_string(),
-            self.id.to_base36(),
+            self.id,
             tiles,
             self.name_line(),
             self.wall_line(),
@@ -248,6 +250,6 @@ mod test {
     fn test_room_walls_array() {
         let output = Room::from(include_str!("test-resources/room-with-walls").to_string());
 
-        assert_eq!(output.walls, vec![10, 15]);
+        assert_eq!(output.walls, vec!["a".to_string(), "f".to_string()]);
     }
 }
diff --git a/src/sprite.rs b/src/sprite.rs
index 1bdc3e8..f42fd2e 100644
--- a/src/sprite.rs
+++ b/src/sprite.rs
@@ -4,11 +4,11 @@ use std::str::FromStr;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Sprite {
-    pub id: u64,
+    pub id: String,
     pub name: Option<String>,
     pub animation_frames: Vec<Image>,
     pub dialogue_id: Option<String>,
-    pub room_id: Option<u64>,
+    pub room_id: Option<String>,
     pub position: Option<Position>,
     pub colour_id: Option<u64>,
     pub items: Vec<String>,
@@ -30,7 +30,7 @@ impl Sprite {
         if self.room_id.is_some() && self.position.is_some() {
             format!(
                 "\nPOS {} {}",
-                self.room_id.unwrap().to_base36(),
+                self.room_id.as_ref().unwrap(),
                 self.position.as_ref().unwrap().to_string()
             )
         } else {
@@ -59,10 +59,10 @@ impl From<String> for Sprite {
     fn from(string: String) -> Sprite {
         let mut lines: Vec<&str> = string.lines().collect();
 
-        let id = from_base36(&lines[0].replace("SPR ", ""));
+        let id = lines[0].replace("SPR ", "");
         let mut name = None;
         let mut dialogue_id: Option<String> = None;
-        let mut room_id: Option<u64> = None;
+        let mut room_id: Option<String> = None;
         let mut position: Option<Position> = None;
         let mut colour_id: Option<u64> = None;
         let mut items: Vec<String> = Vec::new();
@@ -77,7 +77,7 @@ impl From<String> for Sprite {
             } else if last_line.starts_with("POS") {
                 let last_line = last_line.replace("POS ", "");
                 let room_position: Vec<&str> = last_line.split(' ').collect();
-                room_id = Some(from_base36(&room_position[0]));
+                room_id = Some(room_position[0].to_string());
 
                 if room_position.len() < 2 {
                     panic!("Bad room/position for sprite: {}", string);
@@ -118,7 +118,7 @@ impl ToString for Sprite {
     fn to_string(&self) -> String {
         format!(
             "SPR {}\n{}{}{}{}{}{}",
-            self.id.to_base36(),
+            self.id,
             self.animation_frames.to_string(),
             self.name_line(),
             self.dialogue_line(),