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 autmated 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.