At first slowly, and later more quickly and in greater numbers, green meanies would descend down the pit.
You controlled:
Six laser turrets.
One repair robot.
The laser turrets were organised in pairs, pointed directly at each other. You could fire a turret - but if you missed, you would invariably lose the opposite turret.
Meanwhile, the meanies would explode and destroy anything they touched.
If they breached the wall at the bottom of the well, it was game over.
You could repair wall, and turrets, with your repair robot - but that (and firing) took energy. I can't remember what caused energy to replenish.
The game was, needless to say, horribly difficult - mainly due to requiring no fewer than eleven separate keys to be used simultaneously.