You were correct. Can't castle through check. 

Yes you are right, black cannot castle because of the a3 bishop, the black king is crossing the path of the bishop so it will under attack therefore is illegal.

edit: drat you beat me bondiggity, me a slow typist.

sorry I typed something wrong, I typed there Iplayed white and of course he played white, what I meant was I played white and he played black

However, keep in mind that it's illegal if the *king* passes through a square that's attacked. If the rook is attacked, or needs to pass over a square that's attacked, castling is still legal. For instance, in the diagram either player can legally

ok,thnx this was the first time I knew that, thnx again