In Defense, the player fights waves of enemies of increasing strengths and numbers. In the beginning and between the waves there is a so-called Building Phase, where the player gets to buy cities or units to be placed on the map. Some technologies can also be bought to unlock units of greater strengths. The player only has a limited amount of time per turn, defined by the turn counter. When the counter runs to zero, the turn ends and the enemies move, even if the player was not ready yet. The waves last until all enemies have been killed or all cities have been lost. After each wave the player receives 200 gold per city. Between the waves five goodie huts pop into the map randomly and the player may then harvest them for some extra money.
Defense contains some units known from the basic game (such as Trebuchets and Cannons, with increased range though) and some new units such as the Hill Fighter, Forest Warrior and Jungle Warrior. The promotion system from the main game is used in Defense as is. A drastic change from the main game is that stacking units weakens them. Each stacked unit drops the strengths of the units in the stack by ten percent. This may weaken the units up to ninety percent.