Currency

You can earn gold coins by eliminating enemies coming in waves to attack your base.

Colonies, that you can conquer on the Invasion Map, represent an additional source of income. And an important one because they generate gold coins also when you are offline.

Crystals are the second in-game currency type. They are obtained for every 10th defeated enemy wave or for defending your colonies at the Rebellions Map.

If you have troubles winning a battle you should upgrade your troops or purchase heroes (which help you both in offensive and defensive battles) or mercenaries (that cannot be upgraded and help only in offensive battles.