E.g., if you have your king on e1, a rook on f1 and another one on g1… can you castle if the f1 rook moves? Can you castle if the g1 rook moves? Just “kingside” won't tell you the difference.
But if you have a later board state, with two rooks on one side: how do you know which one is still eligible to castle?
Which of course is annoying to implement, but you do already have to keep state on the history of the game to determine if moves are legal, e.g. you can’t castle twice.
> Which of course is annoying to implement, but you do already have to keep state on the history of the game to determine if moves are legal, e.g. you can’t castle twice.
No, that's what the castling rights field in the FEN is for. Once you castle, you zero out both the k and q bits.