The lesson is "Castling" and movement 2/5 where app wants me to castle king with rook. It won't happen and when I hit the solution, app is no longer responds.
What app version? There was a recent update, so if you don't have that version, try upgrading and trying the lesson again.
The lesson is "Castling" and movement 2/5 where app wants me to castle king with rook. It won't happen and when I hit the solution, app is no longer responds.