Stronghold

Strongholds are structures that occur naturally underground, and are primarily important because they house end portals. Strongholds can be located using eyes of ender.

Generation
There is a limit of three strongholds per world (1 per world in console versions). All strongholds are located at random positions in a radius between 640 and 1152 blocks from the origin (the point at x = 0, z = 0, not the world spawn, which may be a couple of hundred blocks away. Strongholds also may extend further in or out of that area). The three strongholds are spawned at roughly equal angles from the center point of the world (that is, each stronghold is in the region of 120 degrees from the others, measured from the origin). The game will not generate a stronghold partially above-ground: any portion above the ground layer will be replaced with air blocks, leaving a cutaway. Strongholds may generate at bedrock level, cutting the bedrock.

As of 1.9, the limit of strongholds has increased to 128 per world. They generate in rings with the strongholds evenly spaced around the circle until the total of 128 is reached.
 * The 1st ring has 3 strongholds within 1408–2688 of the origin
 * The 2nd ring has 6 strongholds within 4480–5760 of the origin
 * The 3rd ring has 10 strongholds within 7552–8832 of the origin
 * The 4th ring has 15 strongholds within 10624–11904 of the origin
 * The 5th ring has 21 strongholds within 13696–14976 of the origin
 * The 6th ring has 28 strongholds within 16768–18048 of the origin
 * The 7th ring has 36 strongholds within 19840–21120 of the origin
 * The 8th ring has 9 strongholds within 22912–24192 of the origin

Strongholds primarily generate underground, and prefer to generate in biomes above sea level but will spawn underwater if necessary. Whole rooms can be flooded if the stronghold is intersected by a mineshaft or ravine. Very rarely, floating rooms can be found in the ocean. Portal rooms (and other parts of the stronghold) can be overwritten by ravines, caves, abandoned mineshafts, dungeons, or parts of the stronghold itself, forcing a player to find one of the other strongholds.

Structure
Strongholds vary in size. They contain several doors and rooms made mostly of stone brick (45%), mossy stone brick (30%), cracked stone brick (20%) and monster egg blocks (5%). Strongholds are lit by enough torches to provide visibility in most areas, though not enough to suppress mob spawns.

Each Stronghold contains several dispersed chests containing a variety of loot.

Rooms
Strongholds feature various types of main rooms. Each room has an entrance, which is either a plain 3×3 opening, a wall with a wooden door, a wall with an iron door (with buttons), or a 3×3 opening with a grate of iron bars leaving a 1×2 opening, and may have exits to other room pieces or dead-end "exits" into stone. Additional "passages" between rooms may exist if their generation overlaps or if the stronghold intersects a cave, mineshaft, or other structure.

Stronghold generation begins with a spiral staircase room with a 5-way crossing at the bottom, with additional rooms being randomly generated from the exits of previous rooms up to a maximum distance of 50 rooms and 112 blocks horizontally. If a portal room is not generated, generation is redone.


 * End portal room: Contains an inactive End portal and a silverfish spawner. In 1.9, extremely rare chance of being active, due to generating with 12 Eyes of Ender. There are two pools of lava in the front sides of the room and a bigger lava pool below the End Portal. There are 16 iron bar windows in the room, and the entrance is always a grate. One per stronghold, and never within 5 rooms of the starting staircase.
 * Libraries (see below). Maximum two per stronghold, and never within 4 rooms of the starting staircase.
 * Large rooms: A room with three exits (four counting the entrance). The center of the room may have various decorations. Maximum six per stronghold.
 * Empty rooms: Has no decoration.
 * Stone Pillar rooms: Has a stone brick pillar with torches on it.
 * Fountain rooms: Contain a fountain in the center, which consists of a stone brick ring and pillar with a water source block on top of the pillar.
 * Store rooms: Dual-level rooms, each 2 blocks high, with a cobblestone center structure joining the two. A lone torch is housed within the structure. A ladder runs up the side wall for access to the second floor, which is made of wood planks and contains a Chest.
 * Five-way crossing: Has up to five exits (six counting the entrance): one across from the entrance down a short stairway made of stone slabs and stone brick blocks, one each to the left and right of the entrance, one on the left side of the upper level reached by a short stairway made of stone slabs and stone brick blocks, and one on the right side of the upper level across a bridge of stone slabs and double stone slabs over the downward stair. A single torch is on the side of the bridge. Maximum five per stronghold.
 * Empty prison cells: A corridor with one wall partially made of iron bars and with two iron doors (in an "open" position, with no buttons). Beyond each iron door are cells with an iron bar wall between them. An exit is opposite the entrance. Maximum five per stronghold.
 * Spiral staircase: A 3×3 spiral stair made of stone brick blocks and stone slabs, leading down, with an exit at the bottom. Maximum six per stronghold.
 * Straight staircase: A 3 block wide staircase of cobblestone stairs leading downwards, with an exit at the bottom of the stairs. There is also an empty alcove under the stairs. Maximum five per stronghold.
 * Corridor: A bare corridor with up to 3 exits: one straight ahead, and optional exits to the left and right.
 * Chest corridor: A corridor with a small altar made of stone brick slabs, with a chest on the altar. One exit, opposite the entrance. Maximum four per stronghold.
 * Corridor turn: A bare corridor with one exit, left or right. May resemble a small room if both the entrance and the next room's entrance have doors.
 * Dead-end corridor: A bare corridor with no exits, although it is likely to intersect other rooms. This room type never has a door or grate at the entrance.

Libraries
Libraries occur in two sizes: Small single-level libraries, and larger duplex libraries. All libraries contain bookshelves, wooden planks, chest(s) and cobwebs dispersed randomly.

Large libraries contain a second level consisting of a surrounding balcony with fences as railings, and fences/torches arranged in a chandelier formation over the center of the room. The two levels are connected by ladders on the wall furthest from the entrance.

Chests are found on the corner of one bookshelf in each library, with an additional chest in the corner of the top floor in duplex libraries.

Trivia

 * There are exactly 233 bookshelves in one large Library Room, which, if mined with a non-Silk Touch axe, will yield exactly 699 books.
 * In Minecraft PE strongholds generate under villages where possible. This is because Minecraft PE has no eyes of ender and thus no way of finding a stronghold.
 * There is an extremely low chance for all the end portal frames in the stronghold to be filled completely, thus allowing the player to access the end even without going to the Nether.