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A defense map is a special Use Map Settings map. In most of them, you start without a base or a limited number of structures, and a variable number of resources (or civilians). Usually, when you kill a certain amount of enemies, you receive resources, or a civilian that represents a dollar to buy special powerful units with.
The objective of the player in these type of maps is just to defend a determined road, killing as many enemy units as he/she can. These are computer-controlled units that run through a certain path where the stationary defenses and units the player have (usually photon cannons, sunken colonies and the like). There are multiple waves of enemy units, each one more powerful than the last, and allowing any through usually has penalties. Commonly the computer units don't attack, but if they encounter with a stack, they will try to destroy it and pass.
These maps are made for multiplayer games and are similar to "tower defense" games.