Posted by: @hermit

 

What self functioning magic?....  

... I see nothing at all in the books to suggest that Narnia has some sort of inherent magical protection, apart from the Tree of Protection itself, which was a specific deterrent against Jadis. If such protection originally existed why would a magical apple tree have been needed in the first place?

Well, take into account that it was my best attempt at providing a fun answer to an unanswerable question...

But yes, I was referring to The Tree of Protection.

King Frank was appointed as King on the basis that he did not need to worry about providing a military defence for Narnia, given that the Tree of Protection would repel any attempt by Jadis simply by virtue of magic. 

By all means you can pick as many holes in that theory as you like, as ultimately its just a bit of fun. 

Like we all gave the "correct" but fairly boring answer to the original question straight away (that Lewis was writing a simple children's story) therefore I think all we've really got left is to try and think of whatever fun, silly and wacky ideas we can in order to make things more interesting.