Simple and clear strategy. In the bowels of the earth, dug by the tunnel, trying to break the evil creatures. That avoid this, it is necessary to route the enemy to place defensive towers. Each tower is unique, it has a cost, damage, range and rate of fire. When you start a budget, you can only afford the cheapest of the tower, but after the first fight will change. For the elimination of opponents, you'll get money, which can buy new towers and improve existing ones.