Look at this game.
Look for the game called "the last stand" and try to see what is different with ur games engine. People want the tank/turret to move around on the screen, and enemies should only come from one direction. Make it have more than one character involved. definetly more upgrades. try to look at other good games and see why they are so good.this game just had the right idea, but just lacked a lot.If u can make this, you can make a lot more.