Berzerk is basically a kill or be killed shooting game that takes place within a maze. It also spawned a large number of clones and influenced everything from Robotron to Smash TV to Loaded to Machine Hunter. The enemy robots in Berzerk start off quite dumb, but as the game progresses they begin to move swiftly and even fire back at you. There are two exits in each screen and you're permitted to prematurely exit without killing every enemy, but at the cost of bonus points. Take too long to clear or exit the screen and the infamous Evil Otto appears. That's a clear indication that you had better get the heck out of there!
Atari did one of its best arcade-to-home ports with Berzerk, and it's definitely worth a look, if even just as a history lesson.