Stronghold

Strongholds (previously unofficially referred to as ruins) are part of the 1.8 Adventure update. They are a new type of structure that occurs naturally underground, and they contain multiple rooms, corridors, stairs, doors and other aspects, as of the version of 1.8. 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. Unlike other 1.8 structures, only three Strongholds spawn per world, but it was a known glitch to spawn only 1. Also found in strongholds are Doors (wood and iron), Bookcases, and Buttons. Some treasures found in strongholds include Paper, Books, Coal, Gold Ingots, Iron Ingots, Compasses, and Red Apples.

Stronghold
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. You can find golden apples in some chests Strongholds can be big depending on the map seed.

1.8 Beta
Strongholds appear in the Beta 1.8 version. They can appear in worlds created prior to 1.8, provided the player has not yet visited the areas where a Stronghold spawns. While the world map is effectively infinite, the number of strongholds is limited to three per map (will probably be infinite in 1.9),.

Stronghold appearance
Strongholds vary in size (though they can be very large). The Strongholds are Labyrinth-like and contain several doors and rooms. The strongholds are mostly made of Stone Brick, Mossy Stone Brick, and Cracked Stone Brick. Doors of both types with buttons and Iron Bars are also located here with stone slabs forming stairs and central pillars of some rooms. Torches are scattered throughout strongholds. Some rooms contain Chests with various items.

The majority of a stronghold is composed of many random hallways, spiral staircases and regular stairways. There are both wooden and iron doors, the latter of which are operated by buttons and may be either open or closed. Hallways can feature decorative arches made of iron bars. Hallways also seem to end in dead ends blocked by regular stone more frequently than would be expected by chance.

Strongholds also feature various types of main rooms, such as Prison Cells, Libraries, Storage Rooms, and rooms which function as four way intersections with a central fountain or column with torches mounted. There are also unusual rooms with a staircase descending under an arch or bridge against the wall, with another staircase to one side leading up onto this bridge (which rarely allows you to reach anywhere else). While the overall placement and number of these features in the stronghold is random, the features themselves seem to always take the same forms. One exception is, at the bottom level of a stronghold, your player is only able to see 10 blocks ahead.

Prison cells seem to be generated in pairs in the wall of a hallway and feature open iron doors without buttons, and are empty.

The libraries are two-floored rooms containing Bookshelves, Fences, and Wooden Planks arranged in an upper balcony over a lower floor filled with bookshelves and Cobwebs, connected by a ladder on the far wall from the entrance. Despite the fixed arrangement of the libraries themselves, the cobwebs seem to be placed randomly within.

All treasures mentioned before are accounted for in the prerelease. The Map Generator frequently generates strongholds underground, often unexposed to the surface and only visible within caves, possibly even underwater caves. The Map Generator is apparently incapable of discriminating between strongholds and standard blocks, as map generation features such as Underground Ravines and Abandoned Mine Shafts cut indiscriminately through strongholds, often making the strongholds difficult to navigate. It is unclear whether this functionality is intended or not. Insofar as the 1.8 Beta, the strongholds are vacant except for regular mobs that spawn under normal conditions. Strongholds are the only location where Silverfish can spawn, created when someone mines one of the blocks within. No new NPCs besides this have been added to strongholds as of yet.

Chests can be found on the upper floor of library rooms and on the upper floor of 4-way intersections. Chests in libraries contain paper, books, a compass and sometimes a Map (Item) while the intersection chests contain coal, bread, redstone, iron ingot, Iron Pickaxe and Red Apples, sometime chests can contain a very rare item such as a saddle.

Spawning
All Strongholds are located at random in a circle between 640 and 1152 blocks from the origin, 0/0. Due to the generation algorithm, the Stronghold proper may extend further in or out of that circle. The three strongholds are spawned at equal angles from the center point of the world (that is, each stronghold is 120 degrees from the others, measured from the origin.)

There is a tool called Stronghold Finder that gives you the coordinates of the three strongholds in your world.