Mike Nelson wrote on Tue, Jul 2, 2002 11:29 PM UTC:Excellent ★★★★★
The castling rule might not quite be sufficient. While it is simple enough,
there are some cases to consider:
1. The rule as stated is fine for a simple colorbound piece such as a
bishop. The bishop moves a-file to c-file -- a square of the same color
that it could reach by legal moves on an empty board.
2. What about an alfil starting on a1? It could never reach c1--but it can
by castling. Similarly a wide half-knight (hN) on a1 could never reach c1,
even though it is not 'colorbound' (but can't reach all 64 squares).