Castle Avarius is the former ruling abode of Ludo, located in the protected Groundlands of Mewni. It was his family's ancestral home until Ludo locked them out, after which it became home to the many monsters and demons that worked under Ludo, and where they organized plans and devised strategies to steal Star Butterfly's wand.
The castle gets blown up in "Storm the Castle" after Star uses the whispering spell to destroy her wand.
Castle Avarius is located in a dark, mountainous region of Mewni with dead trees and skeletal remains everywhere. The castle itself has multiple towers, white flags, a gargoyle above the door, and dead plants that have grown out of the crevices in the castle.