u/Antileous-Helborne

I've spent way too long building (since before the before times) a 3D chess engine, and it's finally playable against a decent AI:

ch3ss.app/play

The website is totally free, no signup, runs in the browser.
I haven’t really tested multiplayer over the web but the AI and hot-seat modes work well. Maybe one day the app will launch with a purchasable skin set.

Anyway, Ch3ss :

The board is three stacked 8×8 levels (bottom / middle / top), aligned on the same files and ranks. White starts on the bottom level, Black on the top, and the middle level starts empty. Within any single level it's just normal chess, but now pieces can also change levels.
King — moves one square as usual, and can change at most one level per move (including straight up/down to the aligned square).
Rook —slides normally on its level; can step to an adjacent level one square orthogonally (or straight up/down); and can cross both levels (1↔3) up to two orthogonal squares if the middle square in the path is empty.
Bishop — slides diagonally on its level; one diagonal step to an adjacent level; two diagonal squares across both levels if the middle square is clear.
Knight — normal L-shape on its level; normal L-shape in 3D: jumps two orthogonal squares to an adjacent level, or one orthogonal square across both levels. Still ignores blockers.
Pawn — level movement is forward-only (White goes up, Black goes down). It can step forward, step up a level in place, or do both at once; double-pushes can also climb two levels from the start. Captures are diagonal-forward on the same level or to the level above/below.

The overall theme is that is feels like a natural extension of chess into the vertical dimension.

Castling, en passant, and promotion all work, including interlevel castling (king crosses two levels, rook lands on the middle). One fun consequence of 3D: a lone rook, bishop, or knight can't actually deliver mate, so some endgames are dead draws that aren't in normal chess.

I think it’s pretty fun, if not hard, and I hope you do to.