Butterfly Castle is the home of the ruling family of Mewni: King Butterfly, Queen Butterfly, and Star Butterfly. It later becomes the residence of Marco Diaz. The castle is significantly larger than other buildings in Mewni, possessing numerous towers of varying sizes that appear to be built into a mountain. Several railways and tunnels connect the towers to one another.
Known areas/chambers inside the castle include the throne room, the Tapestry Room (which Star refers to as the "Grandma Room"), the dining hall, the King and Queen's bedchambers, a dungeon area, and the sub-basement/laundry room.
In "Sweet Dreams", Eclipsa reveals that the castle has several hidden passageways, one of which is behind the portrait of a sea captain in Star's room.
In "Conquer", Meteora Butterfly attacks the castle and destroys large portions of it, including the tapestries in the "Grandma Room" (save for the one of her mother).