Arrow keys or a swipe to move, space or a touch to fire. Rows of invaders shuffle sideways, drop a line at the edge, and come down. Four bunkers sit between you and them, and they erode from both sides: their bombs take chunks out of the top and your own shots take chunks out of the bottom.
Each level makes them faster and the bombs more frequent. The thing the original understood, and this rebuild keeps, is that the tempo increase is not a setting but a consequence: fewer invaders means the machine can move them more often, so clearing the screen is what makes the last one impossible to hit. It works on a phone, which the cabinet did not.