There are six currencies type in Olympus Rising. Each currency is used in different ways either to buy, unlock, build, or give an advantage in game.
Gems are the premium currency and therefore the most valuable in Olympus Rising. They are used to upgrade the Alliance Hall of Uranus, unlock additional unit/power slot for your Heroes, cast invocations and many other things. They can be bought with real money, be earned through quests or be found in chests. If someone uses invocations against your base, you earn some gems as well.
Gold is the standard currency in Olympus Rising. It is used for upgrading buildings, decorations, defensive structures, units, waves and powers. You can also buy item chests with gold.
Wisdom is the second standard currency in Olympus Rising. It is used for upgrading buildings, decorations, defensive structures, units, waves and powers.
Ambrosia is needed to attack islands. Ambrosia is produced by the Ambrosia Temple and stored in the Ambrosia Silo. To increase the ambrosia production you can leave some Heroes on ambrosia islands. You can also buy ambrosia with gems.