You get coins for each monster you kill. With these, you can buy more turrets. You can also improve the range and attack of turrets by upgrading them. Note that the monsters also wear down the turrets. Thus, you must also budget for their replacement.

Unlike other tower defense games, you have limited placement options. These places are often strategic choke points. The type of turret you place in these places matters. Turrets with short ranges belong closer to the edges. Long-ranged ones belong in the middle. The game’s replay value comes from trying out different configurations of turrets.