Vanish Cap Under the Moat

Vanish Cap Under the Moat is the Cap Switch stage where you unlock the Vanish Cap. It is unlocked by draining the castle moat. This is the only course visited from the Castle Grounds. This stage is normally only accessible after you have drained the moat by ground-pounding the two pillars in the Basement, though it is possible to visit it before any other course by exploiting glitches.

This course consists of a bottomless black pit surrounded by towering walls decorated with a gray and taupe triangle pattern. The level makes use of see-saws and moving platforms. The only enemies here are amps and fire spitters.

One of the castle's secret stars can be obtained in this course by collecting 8 red coins.

Behind The Wall
According to some reports, there is a secondary Power Star in Vanish Cap Under the Moat that requires the use of the Vanish Cap to walk through a wall to locate the star. The wall is usually transparent, though has been reported to be entirely opaque, potentially due to the A.I. failing to change the wall's appearance after implementing this new star.

Trivia

 * Vanish Cap Under the Moat is one of the few stages to have a known physical location in relation to Princess Peach's Castle.