Howl's Castle

Howl's magical castle is where the movie and first book in the trilogy derive their name from, and serves as a major setting in them.

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

Within
The inside of the castle is not at all what anyone might expect. In fact, it is not the inside of the castle at all. Upon entering the castle through its magical door, one actually enters into the house in which Calcifer is in. At first, this is the small cottage in Porthaven, though is moved to the hat shop house in Market Chipping. 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. There are however two windows that are not attached to the Porthaven/Market Chipping house. 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.

Outside appearance in the Movie
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). This could be a reference to the windows in the original novel that opened to places other than Porthaven/Market Chipping. The castle, is quite different in the movie. The movie version depicts the castle as a hodgepodge of houses, mechanics, and even cannon stations that form a what appears to be a walking mechanical creature with chicken-like legs and a huge, frog-like face. It retains the four smoke towers. The house is seen as ugly from the Wastelands as commented by grandma Sophie.

Wizard Howl's Moving Castle
As mentioned above, Howl does indeed have a moving castle. Powered and steered by Calcifer, it roams the hillsides around the Chipping Valley, sometimes going as far as to the border of The Waste. How it moves is never precisely described in the books. In the movie, the castle itself is shown to have short, spindly legs that it uses to walk around.

The Magic Door
What truly makes Howl's castle a moving castle, however, is its unique front door. There is a small dial near the doorknob, and where the door leads to when opened depends on what color the dial is turned to. This allows the residents to jump many many miles nearly instantly.