You have to clear everywhere from the alien creatures that are occupying them. But fighting all of these aliens alone is not easy, so you will die a lot! But every time, you will get some XP, level up and gain powerful passive powers that will help you in the game. The angels are on your side too and every few levels, they will come to your aid and give you a free boost. You keep your coin when you lose and you can use it to upgrade your gun, armor and etc. to make your character stronger. So your death is not totally in vain and you will become more powerful every time!