This is very cool, I'm enjoying it.

One small note: I tried to castle, and was told "Castling is not allowed because your king was previously in check."

This isn't one of the rules of castling in regular chess. [1]

Also, it would be nice to implement the three-same-position draw, since I got trapped in a cycle of just moving my king back and forth forever.

1. https://en.wikipedia.org/w/index.php?title=Castling#Requirem...

So in this version the bishops can change the color they can move on?

The corners with 3 and 5 neighbour have a special mark. What does that mean?

Can you share how the board is generated or it's part of the secret special sacuce? I think a blog post about the generation of the borard with a few nice graphics can get a lot of traction here.

Very cool! Looking forward to playing it.

I suggest to minimize visual clutter, because any new player's brain will be overloaded trying to figure out a board, and less is sometimes more.

I'd remove the forward direction triangles where they can be unambiguously inferred from the baseline. A lower grain contrast wood texture might be a good idea too.

You mentioned you sometimes use five tile colours but you can probably improve it: yes, the four colour theorem guarantees that :)

Making Go Twist is a bit more straightforward, since the rules work unchanged for any undirected graph. Such as this diamond lattice [1] in which inner points still have 4 neighbours.

[1] https://en.wikipedia.org/wiki/Go_variants#/media/File:Virgil...

Pawns should probably have a piece that indicates their orientation. As far as I can tell it is impossible to discern which way a pawn moves without querying the ui.