diff --git a/Shared/Debug.cs b/Shared/Debug.cs
index 89a3448..741e0a0 100644
--- a/Shared/Debug.cs
+++ b/Shared/Debug.cs
@@ -28,9 +28,9 @@ namespace SemiColinGames {
          
     public static bool Enabled;
     // This is a LinkedList instead of a List because SetFpsText() adds to its front.
-    static LinkedList<string> toasts = new LinkedList<string>();
-    static List<DebugRect> rects = new List<DebugRect>();
-    static List<DebugLine> lines = new List<DebugLine>();
+    static readonly LinkedList<string> toasts = new LinkedList<string>();
+    static readonly List<DebugRect> rects = new List<DebugRect>();
+    static readonly List<DebugLine> lines = new List<DebugLine>();
 
     static Texture2D whiteTexture;
 
diff --git a/Shared/Player.cs b/Shared/Player.cs
index 4635f7a..b67500a 100644
--- a/Shared/Player.cs
+++ b/Shared/Player.cs
@@ -11,9 +11,9 @@ namespace SemiColinGames {
     private const int jumpSpeed = -600;
     private const int gravity = 2400;
 
-    private Texture2D texture;
     private const int spriteSize = 48;
     private const int spriteWidth = 7;
+    private readonly Texture2D texture;
 
     private Point position = new Point(64, 16 * 14);
     private int jumps = 0;
diff --git a/Shared/SneakGame.cs b/Shared/SneakGame.cs
index bd2a384..c9775c2 100644
--- a/Shared/SneakGame.cs
+++ b/Shared/SneakGame.cs
@@ -7,7 +7,7 @@ using System.Collections.Generic;
 
 namespace SemiColinGames {
   public class SneakGame : Game {
-    GraphicsDeviceManager graphics;
+    readonly GraphicsDeviceManager graphics;
     RenderTarget2D renderTarget;
 
     SpriteBatch spriteBatch;
@@ -16,15 +16,15 @@ namespace SemiColinGames {
     bool paused = false;
     IDisplay display;
 
-    History<Input> input = new History<Input>(2);
+    readonly History<Input> input = new History<Input>(2);
 
-    FpsCounter fpsCounter = new FpsCounter();
+    readonly FpsCounter fpsCounter = new FpsCounter();
     Texture2D grasslandBg1;
     Texture2D grasslandBg2;
 
     Player player;
     World world;
-    Camera camera = new Camera();
+    readonly Camera camera = new Camera();
 
     public SneakGame() {
       graphics = new GraphicsDeviceManager(this);
diff --git a/Shared/World.cs b/Shared/World.cs
index 1ea056f..f18a851 100644
--- a/Shared/World.cs
+++ b/Shared/World.cs
@@ -17,8 +17,8 @@ namespace SemiColinGames {
   }
 
   class Tile {
-    Texture2D texture;
-    Terrain terrain;
+    readonly Texture2D texture;
+    readonly Terrain terrain;
     Rectangle position;
 
     public Tile(Texture2D texture, Terrain terrain, Rectangle position) {
@@ -84,14 +84,14 @@ namespace SemiColinGames {
   class World {
 
     public const int TileSize = 16;
-    int width;
-    int height;
-    Tile[,] tiles;
+    readonly int width;
+    readonly int height;
+    readonly Tile[,] tiles;
 
     public int Width { get; }
     public int Height { get; }
 
-    string worldString = @"
+    readonly string worldString = @"