Keys "s"=save "l"=load "b"=back once
Royalty passes between king and guard, whichever is first if you go a1 to a8, then b1 to b8, and so forth. I allow castling if and only if the original king has always been the royal one.
Play it here.
These are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!