This could be a reference to the windows in the original novel that opened to places other than Porthaven/Market Chipping. In the movie, the house and the castle seem to be interchangeable, with some doors and windows opening to the castle, while others open to Porthaven (or after the move, Market Chipping). One is a tiny window above the door that Calcifer uses to see where he is steering the castle, and the other is in Howl's room, which views his sister's backyard in Wales so he can watch his niece and nephew from Ingary. There are however two windows that are not attached to the Porthaven/Market Chipping house. It consists only of a few small rooms on two floors, a parlor with a fireplace, and an entrance to a small yard where Wizard Howl creates many of his magical objects. At first, this is the small cottage in Porthaven, though is moved to the hat shop house in Market Chipping. Upon entering the castle through its magical door, one actually enters into the house in which Calcifer is in. In fact, it is not the inside of the castle at all. The inside of the castle is not at all what anyone might expect.

The four towers often billow smoke, and the harder Calcifer works, the more smoke is issued. Only the third door can be reached, which although is the back door of the castle, is the front door of the house within. However, two of these doors cannot be approached at all, as though there were an invisible barrier blocking the way. The moving castle appears from the outside as a tall, off-balanced, black-brick castle with four square towers, and three doors.

Howls moving castle calcifer