Instant Fortress

You can use an action to place this 1-inch metal
cube on the ground and speak its command word.
The fortress is a square tower, 20 feet on a side
and 30 feet high. Each creature in the area where
the fortress appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage
on a failed save, or half as much damage on a successful one. The tower is made of adamantine,
and its magic prevents it from being tipped over.