Game loosely based on the original Spacewar!
Two ships duel in a gravity field. Gravity doesn't affect
the ships themselves (which have spanking new anti-gravity
devices), but it affects missiles launced by the ships.
You can play against the computer, or two players can play
with one keyboard. There is also a Gravity Wars mode, where
the two ships do not move, and the players repeatedly
specify the direction and velocity of their missiles.