Gold is an exclusive resource used by both factions. It can only be obtained by purchasing it with real money through Prime World's official website, by redeeming Gold promotion codes, or daily from a Treasury.

Gold is used to unlock new Heroes (even if you don't meet their castle level requirements).
It can also be used to purchase unique buildings, like the Dog House & Cat House, and to obtain "Golden Age", which boosts the productions from all your buildings and improves your rewards from battles.