In no particular order...
BUG: The wrong color starts the game. White (the side with the queen on a white tile, default Red Player 2) should start.
BUG: A king's side castle currently moves the rook a space too far.
I'd throw in a unique text for en passant, just so folks who aren't aware of the rule know what just happened.
It'd be nice if there was a warning for when a king is put in check.
BUG: Just moved a knight onto the kings space, and before victory was declared the game allowed me to promote the unit as though it were a pawn.
^RELATED: Just moved a knight onto the opponents king space (no king present) and was offered a chance to promote. I declined, and next turn moved a pawn forward a space (Bishop 4) and was offered teh chance to promote the random pawn. After further testing, it appears that selecting the pawn on the opposite side of the board, deselecting it, and then moving any other pawn offers a chance for the second pawn to be promoted.
^RELATED?: As red, moved the rightmost rook left one space to take a blue rook. Was offered chance to promote.
BUG: Any piece on a kings starting position gives text allowing the option to castle. Was not able to castle in limited testing.
It would be amusing if units wore more than one bishops hat if they killed multiple bishops. Perhaps having them wear a Pope Hat instead if they kill 2, and a Pope Hat on top of a bishops hat if they kill 3 would be an interesting Easter Egg.
Whee bughunting!