Space War may be the most important computer game ever. The first version was developed for the PDP-1 at MIT in 1960. The game has been under essentially constant development since.
The first CRT display was a converted oscilloscope used to play Space War. The first trackball (and thus, the first mouse) was a Space War control at MIT. It is said that Ken Thompson salvaged a PDP-1 and created a new operating system, now called UNIX, so that he could play Space War.
The gameplay was inspired by E. E. “Doc” Smith’s Lensman novels. Two players go head-to-head, each controlling a ship in interstellar combat, trying to blow the bejeezus out of each other. There is a sun in the center of the playing field that exerts an inverse-r-squared force on all objects on the screen. A talented player can aim torpedoes such that their trajectory is deflected by the sun’s gravitational force until it intersects with the other player’s ship.