One of the obstacles the player will face on each stage is Defense Units (DU). Every Map the player encounters has a set number of Defense Units. These units impose a limit on the amount of towers the player/team can summon. Typically the stronger the tower, the more defense units required to summon it. You can click on the images to see more detailed specifications for each level. DU can be saved by placing defenses where several paths meet, reducing the number of defenses built, but this has a cost; the defences will be closer to the crystal, so if the enemies, of which there are enemies from both combined paths, destroy them, which is more likely due to the larger crowds of attackers, you have less time to plug the gap before they reach the crystal.