My cats also never, ever touched their cat castle, scratching box or tunnel, yet they will play for hours or sleep in a cardboard box or washing basket with clothes in it.
My cat loves his cat castle AND any old box he finds but one of his favourite places to sleep is inside the washing machine when it has a couple of old bits of dirty washing I've chucked in.
My cats hate cat houses/toys etc. They much prefer the big cardboard box my bike came in out the back to sleep in :\ and my lounge is way better for scratching.