Age of Empires Online will be a free download when it releases on August 16, supplemented by downloadable content packs promised on a monthly basis. Today Microsoft Game Studios and developer Robot Entertainment revealed a few new packs that we'll see hitting by the end of 2011.

The Celtic civilization are "masters of metalworking and crafting," which brings special gear and new units like the Woad Raiders, Druids, and Celtic Swordsmen. The Pro Persian civilization brings Immortals, Persian Archers, and attack elephants, along with new unit gear and Advisor cards and both single-player and PVP battle scenarios. Finally, the Skirmish Hall Booster Pack lets you customize your battle, picking your starting Age, forces, resources, difficulty, and number of enemies.

The previously announced pricing plans run the gamut of pricing. The "Empire Extras" pack, a collection of decorations, costs $4.99. The Defense of Crete Booster is $9.99, while the "Pro Civilization" pack offering premium civilization upgrades costs $19.99. These new Celts and Persians packs are only selling individual civilizations, so without announced pricing, it's hard to tell where they'll fall on the cost spectrum.