Looking for a childhood book-weasels in it!?

Ive tried searching google but nothing is coming up.

The book had a black and dark blue cover. I think it was a sillhouette of a tower/ castle. I think the title could be something like "Weasel Tower".

Basically it's about this kid (there might be many kids) who go into this strange world of weasels and stoats, in a dark and magical kingdom. The stoats/weasels are warriors, patrolling the forest. The kid needs to get into the tower/castle for some magical reason.

That's all i can remember! I loved that book...

If this vague info reminds anyone of the book please help out! I really want to read it again!