Tower Chess

From BoardGameGeek:

Tower Chess is a chess variant that's played on a 6x6 grid with two fewer pawns on each side, without knights, and with all of the pieces being flat tokens rather than three-dimensional pieces. Aside from the size of the game board, the main twist in the design is that rather than pieces being removed from the board, they are stacked. When a piece is moved, only the topmost piece of a stack is moved. Pieces that were once covered can move again once all the pieces that were on top have left. You may "capture" your own pieces by landing atop them. As usual, the player to first capture the opponent's king - in this case, by placing one of their pieces atop it - wins.