The Castle Garden is an anomalous area that takes the place of the Castle Courtyard. The location is similar to the Castle Courtyard in texturing, but doesn't have the Eternal Star statue, the Boos that are usually present, and other normal findings in the Castle Courtyard. The Castle Garden is extended much farther than the Castle Courtyard and is considerably empty aside from the pathway, a flowerbed, and clumps of trees. You can view a video of it briefly being explored here.
Upon entering the Castle Garden, you will be greeted by a view of the pathway with a group of bubbly trees on the left and a flowerbed with some more bubbly trees on the right. The path leads to a sign that states:
"Feel the breeze of the wind and smell the flowers! Enjoy your time out here in the Castle Garden."
A diversion on the path leads to another doorway. This doorway will lead you to the Dark Room. The overall atmosphere of the location is eerie due to there not being many signs of life, aside from the ambient sounds of birds chirping.
Some have suggested that the change of the Castle Courtyard to the Castle Garden occurred after a message appeared during one player's playthrough (see Flying Fortress video) that the Castle had "changed".