Place your defenders on the board and stop the monsters from reaching the exit points. Each level consists of a number of waves and each of these waves is progressively harder. For every monster killed your defenders are awarded gold and experience. Gold can be used to purchase new defenders, level up existing one or pave new areas ready for defenders to use. Once you have a defender of a specific level it unlocks advanced defenders for purchase. These advanced defenders are far stronger than their basic counterparts and often make the difference between success and failure.