Stronghold

Strongholds (sometimes unofficially referred to as ruins) 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. Strongholds spawn just once per world. Also found in strongholds are Doors (wood and iron), Bookcases, and Buttons. Some treasures found in strongholds include Paper, Books, Coal, Iron Ingots, and Red Apples.

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

It is possible that boss monsters will be added to the strongholds. Jeb said in a Twitter post that he plans on adding this feature but that it is not done yet. Strongholds can be big depending on the map seed.