In the early '80s, developers at Stern took the robot from Berzerk and dressed him up in a fuzzy green suit, so he kind of looked like the frog from Frogger. Then they took the mushrooms from Centipede and dressed them up like Space Invaders. Drunk with creative power, they then took the tanks from Robotron and smashed them down into little squares. Then they put their new creations in a world that looked a lot like Zoo Keeper. The "destroy all the hoppers and safely escape the level before the walls crush you" genre was created.
Gameplay: You're a froggy-looking creature with no pants, but you do have a machine gun—talk about art imitating life! You use your machine gun to blast away all the hoppers (who don't hop so much as just sit there) and escape through the top or bottom of the screen before the walls close in on you. Each level is named for a different type of monster that's out to kill you, like the Eyeball Room or the Sickle Room. In addition to your gun, you have some smart bombs, which instantly destroy all the monsters and briefly stop the walls from moving.