In variants where there is more than one rank behind the Pawns, and many strong pieces, castling directly behind the Pawns is often a very bad move. In general one could say that in such variants the castling rule is only added out of conservatism, but is as useless as adding a rule that at any time you can permanently remove your Queen from the board instead of making a normal move. No one in his right mind would ever do it. Even in the central files, a King on the back rank sheltering behind by a wall of Pawns plus a wall of pieces is usually a lot safer than a King directly behind the Pawns.
So the first issue to consider is whether it is really a good idea to force an engine to make poor moves just because the rules allow those. That being said, if it is really such that castling is good, the usual way is to discourage moving an uncastled King not by Piece-Square Tables, but by giving a hefty bonus for the possession of castling rights. (Which should reflect the PST and other positional score the King would get in the castled position. If they were higher the engine would never castle, because he would think it more important to preserve the rights to do so.) In KingSlayer I always evaluate the quality of the Pawn shield in the castled locations, and derive the value of each castling right from that. These are then devaluated a little depending on the number of pieces that still block the castling. And the total value of the rights is that of the best right plus a factor smaller than one times the other.
In variants where there is more than one rank behind the Pawns, and many strong pieces, castling directly behind the Pawns is often a very bad move. In general one could say that in such variants the castling rule is only added out of conservatism, but is as useless as adding a rule that at any time you can permanently remove your Queen from the board instead of making a normal move. No one in his right mind would ever do it. Even in the central files, a King on the back rank sheltering behind by a wall of Pawns plus a wall of pieces is usually a lot safer than a King directly behind the Pawns.
So the first issue to consider is whether it is really a good idea to force an engine to make poor moves just because the rules allow those. That being said, if it is really such that castling is good, the usual way is to discourage moving an uncastled King not by Piece-Square Tables, but by giving a hefty bonus for the possession of castling rights. (Which should reflect the PST and other positional score the King would get in the castled position. If they were higher the engine would never castle, because he would think it more important to preserve the rights to do so.) In KingSlayer I always evaluate the quality of the Pawn shield in the castled locations, and derive the value of each castling right from that. These are then devaluated a little depending on the number of pieces that still block the castling. And the total value of the rights is that of the best right plus a factor smaller than one times the other.