It's post-apocalyptic time and few humans have survived. But those who did are now looking for subjects of battle. There was a tower and some people living peacefully in it, until some other people came and look for battle. So the people living in the tower have to defend themselves. Help them expand the tower and build some awesome weapons with which can defend themselves. Just aim and shoot all the enemies in order to don't let them get to the tower. We must defend the tower, because it's all what remained after the apocalypse.