GTA: Wasteland Siege
In a far away place a tower was erected and enemies began appearing demanding that they get the tower or else. You never agreed to give them the tower, so the war began. The whole idea here is to defend the place, so you need to erect some firearms in your tower. You would start with a few money to erect your tower. You earn money with each kill and you can either upgrade your existing tower or buy an upgrade with the money that you make. The game becomes harder and more difficult as you make some progress.