An instant fortress was a magic item that appeared as a small rust-streaked iron cube about an inch across. When placed on the ground and given a command word, it expanded into a square fortress of Adamantine, twenty feet to a side and thirty feet tall, with walls that extended another ten feet below the earth. The walls were lined with arrow slits and topped with battlements, and the structure could not be tipped or forced open except by its owner. The fortress contained two floors, a trap door to the roof, and space to house up to twenty people, though it provided no food, water, heat, or air of its own.
The adamantine walls resisted most magic and were impervious to normal weapons, though siege engines could damage them. An instant fortress could not be repaired unless completely melted and re-forged. Once raised, it remained a permanent structure until dismissed with a command word, which only worked if it was empty. The fortress posed a hazard when it expanded, as its sudden growth could crush those nearby.
Rare and highly valued, it was a tool favored by travelers and Adventurers who needed a secure defense in hostile or unsettled lands.