Strongholds are large, underground structures and can be seen as a long abandoned castle. The stronghold contains many different rooms, ranging from simple empty rooms, to prisons and libraries. The stronghold also contains an End portal, which is guarded by a silver fish spawner. These silverfish will enter cobblestone, stone and all stone brick blocks around them. Those stones will release the silverfish when broken, which may then call all the other silverfish in the surrounding area, which could lead to a large swarm of these silverfish you’ll have to deal with.
The portal is always part of a portal room, though the location of this room is randomly generated. The portal allows a player to travel to the End, but it has to be activated first. This is done by placing an eye of Ender in each frame, up to 12 eyes could be needed.
There are only 3 strongholds per world, which means finding one can be very difficult. However, shooting an eye of ender into the air will cause it to float in the direction of the nearest end portal and thus the stronghold. This means that with enough eyes of ender, anybody should be able to find a stronghold. Note that strongholds will always spawn between 640 and 1152 blocks away from the 0,0 coordinates, so there’s no point in traveling the other way to find a second or third stronghold.
Besides the large amounts of mossy stone brick, cracked stone brick and normal stone bricks you can mine from a stronghold, strongholds also offer some other resources, some are found in chests while others are part of the stronghold.
Chests can be found in the libraries, either on the second floor or on top of bookshelves, and they’re also found in the corridors. Chests can contain one or more of the items listed below, but note that library chests will only contain paper, maps, compasses or books.
You can also find iron bars, iron doors, buttons, books (from bookshelves), torches and various blocks, like wooden planks and fences through the strongholds.