You can use an action to place this 1-inch metal cube on the ground and speak its command word. The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it. The fortress is a square tower, 20 feet on a side and 30 feet high, with arrow slits on all sides and a battlement atop it. The tower has AC 15, 100 hit points per side, and immunity to nonmagical damage. Only you can dismiss the fortress.