[[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?

[[WMG: The books have their own special kind of magic.]]
By that I mean that the books change history. The things that are written in there change things in real life. For example, why are there dinosaurs without feathers in the first book? That's because the book changed history and you see that instead of anything else.

[[WMG: You can only go into the book's contents, but not actually travel in time.]]
That's why in the first book there's dinosaurs without feathers. Because the book showed them like that, instead of what they really are.
That would also explain why, for example, in ''Eve of the Emperor Penguin'', no one made a big fuss on the news about two children suddenly showing up on an Antarctic research base: because they only traveled into the book, not within their own world.

[[WMG: InUniverse, the versions of ArthurianMythology where Morgan is evil are an attempt by her enemies to ruin her reputation in history.]]
Upon first meeting Morgan, Jack says "They said you were a witch.", indicating that tales with a negative portrayal of Morgan exist in this world, and are widespread enough that they're in books read by 8-year-olds. However, from what we've seen in the series, we know that Morgan is actually a BigGood who has helped save Camelot on several occasions and organized the recovery of several powerful artifacts like the Diamond of Destiny and the Wand of Dianthus that might otherwise have fallen into the wrong hands. As ''Blizzard of the Blue Moon'' showed, some evil wizards are capable of traveling to the mundane world's past, so it's possible that one of them organized a SmearCampaign against Morgan out of spite. (And/or to encourage people who encountered her or people who claimed to work for her to refuse help. Obviously it didn't work, as Jack and Annie usually have very little trouble finding help when they need it.