Chess Online

As was made clear in all the comments on puzzle puzzle #10609, by convention castling is always possible in a chess problem unless it can be proved FROM THE POSITION that it's not possible.

The only way we can know that it's not possible here is because if you click on the white king, the blue dots show that the king cannot move to g1. We only have this feature because Gameknot supply it as an additional aid to show what squares a piece may move to. It's not a feature of the laws of chess.

I'm not saying this as an excuse for the fact that I failed to solve both this puzzle and 10609. I'm just saying that IF I had solved 10609 (which I didn't), and then came here and saw the same position in this puzzle, and tried to solve it the same way (which I didn't), I would feel cheated because there is nothing in the position that proves castling to be impossible.

Nonetheless @sirissac if one does accept this one as a valid puzzle, it's worthwhile in its own right and is arguably more of a challenge than 10609.