This was on /r/programming and now it's here - I'm sorry, but I don't really understand what this is all about. 600 lines doesn't seem ultra-small to me. And Lisp - well sure, not too common a language for games, but it's not as if this was written in a purely functional style or something. In fact, it looks like pretty standard imperative programming to me - it wouldn't look much different in C.

Am I missing something here?