diff --git a/Design-Doc%3A-%60level%60-tool.md b/Design-Doc%3A-%60level%60-tool.md
index 553edca..37b5f87 100644
--- a/Design-Doc%3A-%60level%60-tool.md
+++ b/Design-Doc%3A-%60level%60-tool.md
@@ -35,4 +35,19 @@ health = {
       value = 255
    }
 }
-```
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+```
+
+## .lvl Format
+The .lvl format is output by the `level` tool. It is a binary format ideal for use on a 8-bit or 16-bit game console.
+
+### Header
+4 bytes: Magic number, 'LEVL' in ASCII. This is useful to quickly evaluate problems with bundling assets in your build.
+
+### Map Unit Size
+1 byte: The unit size for each tile. On Sega Mega Drive this is 8, for 8x8 tile.
+
+### Map Tile Size
+1 byte: The width _in number of tiles_ of each entry in the level map. 
+1 byte: The height _in number of tiles_ of each entry in the level map.
+
+(TBD !!)
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