This metal cube is of small size, but when activated it grows to form a metal tower 20-ft square and 30-ft high, with arrow slits on all sides and a machicolated battlement atop it, the metal extending 10-ft into the ground. 

It has a small door which will open only to the command of the owner of the fortress, knock spells notwithstanding. 

The adamantite walls of Daern's Instant Fortress are totally unaffected by normal weapons other than those of catapult type. The whole can take 200 points of damage before the tower collapses. Note that damage sustained is cumulative. 

The fortress cannot be repaired, although a wish will restore 10 points of damage sustained. 

It requires but 1 round to cause the fortress to spring up, but the person or persons nearby must be careful not to be caught by its sudden growth, or else they will sustain 10-100 hit points of damage. 

The door will always be facing the owner of the device when it becomes a fortress, and it will open and close instantly at his command.