[[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 [[BiggerOnTheInside concealed within the geometric appearance]] of a simple treehouse.
[[WMG: It is in the same universe as a particular light novel, or is at least the predecessor.]]
** There was once a light novel series called ''Domitor Leo''. It spanned six books. The adventures are quite similar in fashion to this series; the only difference is that the books involve a disaster and that the characters are being sent by a particular British person working on the Big Ben clock tower. And the origins of the series' happenings are Arthurian!

Obvious already?