Eliminator

A top-down space shooter set within a bounded playfield, where the player maneuvers a ship using rotation, thrust and forward-firing controls (much like Asteroids or Space War). Each stage is populated by automated drone ships (or by competing players), and dominated by the titular enemy: a moving, rotating space fortress surrounded by a lethal force field.

This circular force field instantly destroys anything that smacks against it. Shots fired by the player (and the drones) are non-lethal: the target is simply kicked back across the playfield, bouncing off the walls. The exception is the enemy ship hidden inside the Eliminator - it can (and must) be destroyed by sneaking a shot through the structure's narrow opening. If this isn't done quickly enough, it will emerge and start pursuing the player with deadly fireballs.