Strangely enough, I believed you when you first came. I think you were tricked. I think that someone or something has been controlling the minds of the guards... just waiting for someone like you to come along. Once you came, the plan was triggered, and as long as you succeeded, another rune stone would be smuggled out...
The only thing I can't understand is how the rune stones left the kingdom. The wall has been locked... nothing gets in or gets out...
You said that you performed some kind of spell lock? Perhaps we can find something there.
Go to the librarian that gave you the spell book and see if he an lend us some information.