Alway check, it may be mate... 1.  0-0#  A good beginning!

In one move..mate..! 

I have never seen a 0-0# mate before, anyone know a game where it could be found?

I pretty much tried everything except ......

Just like the answer: O-O That is wierd.

Nd2, Be2 also work.

 Both of those moves allow Kg2.

lol.. there are many mays to mate.. but this one was special.. it's really weird. otherwise endgames generally do not contain castles..

 There is only one way to mate in a single move.

If illegal moves are allowed, then Qe2 is also mate. If not then O-O is the only answer.