The Magic Treehouse/WMG

The Treehouse is a TARDIS

 * Its chameleon circuit is also broken but its stuck as a treehouse in a tree.
 * And ergo, Morgan le Fay is a Time Lord.
 * Alternatively, the Treehouse is primitive TARDIS constructed with Arthurian Era materials and magic that Morgan and others would have known about, hence why it looks, inside and out, like a simple treehouse full of books, instead of a futuristic spaceship concealed within the geometric appearance of a simple treehouse.