The premise appears to be that you are a Robot defending a heap of rubble and dirt, or getting attacked on top of it, however you like to think of it. As you kill enemies and progress through waves, you are able to buy upgrades for you plasma gun (your primary means of disposal), your treads, a placeable (and enhanceable) turret, as well as your shields, armor, and an EMP shockwave for use against heavily shielded enemies. Of course, the waves continue to get harder, with more enemies coming in all shapes and sizes and attack styles, which keeps things interesting.
Once you progress a fair bit into the game, it is fairly difficult even on easy, but if you feel that does not apply to you can go all the way up to Insane difficulty. But, regardless of the difficulty your play it at, it's all very forgiving since you can always continue from the last wave you managed to clear.
