12+13. Matt in einem Zug!
Suppose white can still castle. First white played f6-f7, after that
black played g7xf6, and played his bishop to g1/h2, over e3, then white
played e2-e3. So the white bishop had to go to e8 via c8 and d7. But
then there is no room left to squeeze in all the black pieces and the
white rook into the northwest corner. So white may not castle (and the
bishop entered the southeast corner over e1,f2). So #1 with 1. Rd1!#
(OOO?# illegal).
Solution by Philippe Schnoebelen
