diff --git a/Design-Doc%3A-%60level%60-tool.md b/Design-Doc%3A-%60level%60-tool.md
index de13ccf..bf29331 100644
--- a/Design-Doc%3A-%60level%60-tool.md
+++ b/Design-Doc%3A-%60level%60-tool.md
@@ -27,6 +27,8 @@ Tile layers without `reskit-layer` set are ignored by the `level` tool.
 ## Collision Map
 The boolean property `reskit-collision` set on a Tile Layer to `true` indicates which layer should be used for building the tile collision map. Any non-zero item placed indicates a tile which the player is not permitted to pass through. How you implement this is up to your game engine - reskit will simply save the collision map in a `.bin` file as an artifact (`collision.bin`).
 
+If no layer is defined with `reskit-collision` you will receive a warning on the command line. Generally most maps should include collision data - it is NOT auto-generated!
+
 ### Format of the Collision Map
 The collision map is just an array of 8-bit unsigned integers laid out with the same tile dimensions as your visible layers. 0 is a free area, while 1 is a collision area.