Defense Units (DU) are a resource that limits the number of defenses players can summon. The amount of DU available varies between maps and challenge missions.
Each defense costs DU to summon. Typically the stronger the tower, the more defense units required to summon it. DU can be saved and used more efficiently by building where several paths meet. However, these intersections are often close to an Eternia Crystal, possibly within the attack range of an ogre or archer.
Shown below is the DU capacities on a number of maps. You can click on the images to see more detailed specifications for each level.