Strongholds are structures that occur naturally underground, and are primarily important because they house End Portals. They can be located using Eyes of Ender, and are used to travel to The End. Unlike most structures, they are intended to be difficult to locate through conventional means, providing almost no outward indication to exposed areas, though they are often intersected by cave systems, ravines and abandoned mineshafts. Strongholds vary in size but are usually large and complex, containing multiple rooms, corridors, stairwells, doors and other features. Each also contains serveral dispersed chests containing any number of items that a player may want or need. They can contain regular mobs that spawn under normal conditions, as well as Silverfish that are created when someone either mines one of the blocks within, or finds the End Portal room, which contains a Silverfish Spawner.
Strongholds feature various types of main rooms. While the overall placement and number of these is completely random, the features themselves seem to always take similar form:
End Portal Room - Contains an unactivated End Portal and a Silverfish Spawner.
Empty Prison Cells - These seem to be generated in pairs within the walls of a hallway, and feature iron doors that can be opened or closed.
Empty Rooms - Small rooms generated off the sides of hallways.
Fountain Rooms - These contain a fountain in the center, which consists of a stone pillar with a water source block on top.
Store Rooms - Dual-level rooms with a cobblestone center structure joining the two. A lone torch is housed within the structure, and a ladder runs up the side wall for access to the second floor, which is made of wood planks and contains a chest.
Staircase Rooms - These have staircases made out of stone slabs and stone bricks. There may or may not be a door at the top.
Libraries - Occuring in two sizes, either small single-level or larger duplex, all libraries contain bookshelves, wooden planks, chests and cobwebs. Larger ones 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.