Ooh, this looks interesting.

Only complaint I have is that people will probably tend to stay on the outside of the map, up where the screenshot is taken from. I'd suggest some kind of walls around the central structure, perhaps castle-like.

Oh, and there are some general packaging guidelines to follow, including "remove OS-specific files" (i.e. .DS_store files and the __MACOSX folder). You might also want to give the archive itself a better name. (Read the full guide at the wiki.)

EDIT: After playing around in it, I have a few suggestions: Remove those little devices at the corners, and just clip the walls up to the tops of the corner peaks. (The peaks themselves don't need to be clipped, but it's not a bad idea.) Also, the stairs with the yellow armor are off-center. Try "/outline 1" or just hit 7 (in edit mode), and you'll see the triangles the game uses to generate the geometry. (The bigger the better.) You'll also notice (especially on a new map) that the whole map is made up of eight of the biggest cubes, and they share a vertex at the exact center of the map. You can use this as a guideline for centering things and making sure they're symmetrical.
One more thing: At the base of one of the stairs, there's snow... but not at the other one.