to bwildman: Of course castling is valid in FRC / Chess960. But because an FRC engine should support all valid FRC-FEN positions it automatically would support all variants without castlings, too. Some engines are limited to support a maximum of 16 pieces a color. But Smirf is able to have the complete board filled with pieces. Thus it could support a lot of variants based on 8x8 standard or 10x8 extended Capablanca piece set. May be it would be better to separate Fortress Chess, because it has more than 16 pieces a color?