OCTA-CHESS

The board is 64 octagons (a normal 8×8 chess grid) plus 49 gates — the diamond cells sitting on every interior corner. Every diagonal runs octagon → gate → octagon.

♜ Rook slides orthogonally across octagons — never touches gates, exactly like chess.
♝ Bishop slides diagonally and may stop on a gate. A bishop resting on a gate can leave on the other colour — gates un-bind the bishops, but cost a tempo.
♞ Knight leaps (2,1) between octagons, ignoring gates entirely — the one piece immune to the board, so it's stronger here.
♛ Queen moves as rook+bishop but is heavy: it may pass an empty gate yet not stop on one, and a piece parked in a gate blocks it.
♚ King steps one step: to an adjacent octagon (orthogonally, or diagonally across an empty gate), or onto an adjacent gate. Roomy on an octagon, only 4 escapes on a gate.
♟ Pawn pushes one octagon forward; captures diagonally along the octagon→gate→octagon line — it takes a piece sitting in the forward gate, or, if that gate is empty, a piece on the diagonally-forward octagon beyond it. So pawns defend each other in chains, just like chess. A pawn that captures into a central gate is a permanent outpost.

◆ Warden (optional, toggle in New game) — a gate-only piece that replaces each side's queenside knight and starts on a home gate. It steps one gate at a time along gate ranks/files, never touching an octagon, so it can only capture pieces that are in gates. Because rooks and queens can't land on gates, the Warden is immune to them — only a king, bishop, knight, pawn or enemy Warden can take it. Its job is to seize and hold the gate network.

Only light pieces (K, B, N, P, and the Warden) can rest in gates — heavy pieces can't cement them shut. The fight is for the four central gates. Checkmate wins.