Cool! I just recently implemented a chess engine in ~400 (readable) lines, with all rules, first in Java and then ported to my own programming language "Bau" [1]. This is including a terminal UI. I'll measure the ELO, but I was never able to beat it :-) The castling moves are specially tricky to implement I think. I enjoyed the challenge as well.

[1] https://github.com/thomasmueller/bau-lang/blob/main/src/test...