What about en passant??
Viewing post in the Whole World is in Check comments
Being that pieces take more than one move to kill, en passant didn't fit too much with the design of the game. I thought of including it in some way, maybe specific to one Set or one Unique piece, but so far I haven't come up with a good design for it. Maybe in future versions if I keep updating the game.
Castling was in the first prototype, back then it created a big balance issue with the Russian Set specifically, and it was scrapped. After version 0.3 the gameplay changed completely and I think castling could be viable again. Unfortunately study, work and other projects have put a halt to many features I planned to include in next versions.