Two pawns start facing each other on a nine by nine board. Amber runs to the far row at the top, Slate to the far row at the bottom, and the board is always drawn with your own pawn at the bottom running up. On your turn you either step one square, or drop one of your ten walls into a gutter to make the other pawn go the long way round. A wall is two squares long, walls may not cross or overlap, and no wall may ever shut a pawn out of its row completely: it may cost someone twenty moves, but it may never cost them the game outright. Pawns face to face jump over each other, and if a wall or the edge stands behind the pawn being jumped, the jump folds sideways. The match is refereed by a server that owns the board, so a tampered page can ask to move twice, spend an eleventh wall or wall off the last route and simply be told no. That same server computes the Elo, from the result it saw itself, which is the only reason a rating here is worth anything.