Stronghold

Strongholds are a possible addition to the 1.8 "Adventure" update. They are a new type of structure that seems to occur naturally, and appear to contain multiple rooms, doors and other aspects. Stone Brick, Mossy Stone Brick, Cracked Stone Brick, and Iron Bars were revealed in the same image as being the materials that make up the strongholds.

The name "stronghold" for this feature has been confirmed by Notch in a Google+ post.