The Goal: Creeps enter from the top and from the left. You must stop them from reaching the other side or you will lose lives. You start with 20 lives, and when they are gone the game is over. The Pieces: Towers are used to attack creeps and also to block their path. You buy a Tower by clicking on its image and then clicking somewhere on the map. There are 5 different towers to choose from, each with different abilities. Towers can be improved by buying upgrades. The Money: Gold is earned by killing creeps, especially the boss creeps which come every 7 levels. You spend Gold on buying Towers and upgrading them. The Levels: Each level, or 'wave', contains 20 creeps (10 on easy) and they arrive every 25 or so seconds. They cannot be stopped. Your only defense is to purchase Towers to attack them. Some waves have special abilities like FAST or FLYING. Also, after every 7th wave is a BOSS wave that is especially hard to kill. Keys: 1-6 selects a Tower & 0 deselects. U upgrades the selected Tower. S sells the selected Tower. Holding down Shift allows you to place multiple Towers one after the other. Space to toggle pause.