Assuming you're talking about Super Mario 64, it's a castle tower in the sky that contains the Wing Cap's ! Switch. Upon entering the level, Mario will be granted with a Wing Cap which he can fly around with. Upon landing on the tower, Mario will be able to press the Red ! Switch, which in turn will make all the Red ! Blocks solid. A Power Star can also be found here. This is the only level that Mario enters the stage flying with the Wing Cap. On the other levels, he must triple jump to take off.