Description taken from Arcade version of this game

Zaxxon is a space combat shooter where you play a lone fighter pilot battling through
a heavily armed space station in an attempt to destroy the evil sentient super robot named Zaxxon. Along the way, you must battle enemy planes, avoid fire from ground turrets, and destroy communications equipment and fuel repositories.

Zaxxon features a three-dimensional perspective on a tilted background. Instead of
flying side to side, players appear to be flying diagonally from the bottom left of the
screen to the top right. Because the game is three-dimensional, players can climb and
descend as well as bank left and right. To help players keep their perspective, the plane's shadow is always visible on the ground below.

Enemy planes emerge from the top of the screen and will fire missiles at the player's
plane. Players must return fire to destroy the enemy plane and maneuver to avoid his fire.
The number of enemy planes remaining on a certain level is shown in the bottom right
corner of the game screen.

In addition to fighting off enemy planes, players must also destroy turrets on the ground,
communications satellites, and fuel repositories. Destroying these items will add points
to the player's score. Destroying the fuel tanks will add fuel to the player's plane. A fuel
meter is displayed along the bottom of the game screen.

In the final battle against the Zaxxon robot, the player's plane remains stationary
while the player fires a salvo of missiles into it. Zaxxon returns fire with arm-
launched missiles. Players must avoid Zaxxon's onslaught and hit the robot boss
many times before he is destroyed. ~ Anthony Baize, All Game Guide