Stronghold

Strongholds are possibly Dungeon complexes, containing multiple rooms, doors and other aspects. The only screenshot of a Stronghold is that of an underground ravine cutting into the Stronghold.

Unlike dungeons, Strongholds appear to be built with stone bricks. It is not clear how Strongholds relate to Ruins at this time.

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