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 (which will be changed to infinite in 1.9). 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, Red Apples, Ender Pearls, and Diamonds.

Stronghold appearance
Strongholds vary in size (though they can be very large). They are labyrinthine and contain several doors and rooms made mostly of Stone Brick, Mossy Stone Brick, and Cracked Stone Brick.

Strongholds are frequently generated 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. As of the 1.8 Beta, 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.

The majority of a stronghold is composed of many random hallways, pillars, spiral staircases and regular stairways formed with stone slabs. There are both wooden and iron doors, the latter of which are operated by buttons and may be either open or closed. All doors in Strongholds seem to face the same cardinal direction when they're generated, so some doors are open and some are closed by default. Hallways can feature decorative arches made of iron bars. Hallways sometimes end in dead ends blocked by regular stone. Torches are scattered throughout many rooms. Some rooms also contain chests with various items.

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.

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

The library has two size versions: A small library and a large library. A small library has Bookshelves, Wooden Planks, and a chest. The large library contains more Bookshelves, Fences, Wooden Planks, and torches arranged in an upper balcony with a chandelier over the small library, 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.

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

Spawning
In Beta 1.8, all Strongholds are located at random in an annulus 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.)

As of Beta 1.9 Pre-release 2, Strongholds are still limited to 3 in a world.

As of Beta 1.9 Pre-release 3, a single or pair of glass towers indicate the location of a Stronghold beneath them, extending from the top of the map to the Stronghold brick. Jeb has stated this is because he forgot to remove the debug towers from Beta 1.9 Pre-release 3.

Trivia

 * In 1.9 Pre-Release 4, the Eye of Ender can be used to locate strongholds. You right click while holding the Eye of Ender and it will then be thrown and go into the direction of the nearest stronghold.