Crystals

The game has two “currencies”. The green crystal is used to build your towers along the projected path of the enemies. Green crystals are also used to repair and upgrade your towers/traps. You can also repair your allies' traps/towers by spending your own green crystals. Green crystals can be acquired from chests that replenish after entering Build Phases between waves. These chests have fixed locations, and will be activated once the Build Phase starts. These chests however, seems to be limited to one per character. That means you cannot open a Green Chest of another player (unless that player dropped out in the middle of the Build Phase and he hasn't opened the chest yet) Green Crystals are also dropped by defeated monsters.

Blue Crystals function like your character's Mana or MP. This is consumed when your character is using an ability. Fortunately, using the Heal ability doesn't cost anything at all. Blue Crystals can be collected as enemy drops as well. Because the resources needed for defense-building and abilities are separate, there's no need to sacrifice one over the other when the situation calls for it.