The monsters are back! Stop them from reaching your portal by constructing towers on the path that they travel. In the beginning of the game, you can build four basic towers: a steam tower, an eye tower, a fire tower, or a laser cannon. You can view the damage, range, and fire rate of a tower before building it. Gold is needed to build and upgrade towers; earn gold by killing monsters and sending out waves early. Upon level completion, you will receive crystals that you can use to upgrade a tower type or reduce its cost.