If you are a Xbox Live Gold member you are treated to two free games on 1st and 16th of every month. Civilization Revolution & Dungeon Defenders come free to Xbox Live Gold members for the month of March.
Civilization Revolution (Normally US $29.99) will be free for Xbox Live Gold members through March 15th . Then on March 16th, Xbox Live Gold Members can download Dungeon Defenders (Normally US $14.99) for free.
In case you want to buy Xbox Live Gold membership, you can avail of $20 discount offer now at Microsoft store.