Cyrstal are used for economy in the game.

Crystals are used for:

  1. Upgrading Good Units
  2. Upgrading Buildings
  3. Unlocking Good Units or Buildings before you reach the required Stage
  4. Crafting Runes

Crystals can be obtained from:

  1. In-app purchase
  2. Finishing a new Stage in Singleplayer Campaign (2 to 3 Crystals)
  3. Replaying a Stage in Singleplayer Campaign (1 Crystal)
  4. Playing Multiplayer Co-Op (Crystals Dependent on time)
  5. Selling Runes (1-4 Crystals)
  6. Playing Siege Mode (0 to 3 crystals depending on time. Also depends on whether you win or lose)
  7. Playing Endless (level 31) in the Campaign  (0 to 3 crystals depending on time)