diff --git a/src/main.rs b/src/main.rs
index 1610554..f8a38d0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -79,9 +79,27 @@ fn model(app: &App) -> Model {
         ..Default::default()
     };
 
+    let boundary = app.window_rect();
+
     /* Create the ground. */
-    let collider = ColliderBuilder::cuboid(100., 10.)
-        .translation(vector![0., -200.])
+    let collider = ColliderBuilder::cuboid(boundary.w(), 4.)
+        .translation(vector![0., boundary.bottom()])
+        .build();
+    engine.state.colliders.insert(collider);
+
+    /* Create the walls. */
+    let collider = ColliderBuilder::cuboid(4., boundary.h())
+        .translation(vector![boundary.left(), 0.])
+        .build();
+    engine.state.colliders.insert(collider);
+    let collider = ColliderBuilder::cuboid(4., boundary.h())
+        .translation(vector![boundary.right(), 0.])
+        .build();
+    engine.state.colliders.insert(collider);
+
+    /* Create the ceiling. */
+    let collider = ColliderBuilder::cuboid(boundary.w(), 4.)
+        .translation(vector![0., boundary.top()])
         .build();
     engine.state.colliders.insert(collider);