Meisterwerke

Suppose black can castle both king- and queenside. Then his last move was either b7-b5, d7-d5 or f7-f5.

• Last move was b7-b5: #3 with 1. cb6 Ra4 2.Rh8 Qh8 3. Qh8# 1. .. Ke7 2. Bb4 Kd8 3. Ra8#.
• Last move was d7-d5: #3 with 1. cd6, and black can't defend against 2. Rh8 Qh8 3. Qh8#.
• Last move was f7-f5: #3 with 1. gf6, and black can't defend against the two threats 2. Rh8 Qh8 3. Qh8#, and Ra8#.

Suppose now black can't castle queenside. Then #3 with 1. Bd6!, and (again) black has no proper defense against 2. Rh8 or 2. Ra8.

And the last case: Black can't castle kingside. Then #3 with 1. Bf6!

Solution by Philippe Schnoebelen