This metal cube is small, but when activated by speaking a command word it grows to form a tower 20 feet square and 30 feet high, with arrow slits on all sides and a crenellated battlement atop it. The metal walls extend 10 feet into the ground, rooting it to the spot and preventing it from being tipped over. The fortress has a small door that opens only at the command of the owner of the fortress—even knock spells can’t open the door.
The adamantine walls of Daern’s instant fortress have 100 hit points and hardness 20. The fortress cannot be repaired except by a wish or a miracle, which restores 50 points of damage taken.
The fortress springs up in just 1 round, with the door facing the device’s owner. The door opens and closes instantly at his command. People and creatures nearby (except the owner) must be careful not to be caught by the fortress’s sudden growth. Anyone so caught takes 10d10 points of damage (Reflex DC 19 half ).
The fortress is deactivated by speaking a command word (different from the one used to activate it). It cannot be deactivated unless it is empty.
Source: Dungeon Master's Guide