Hard to say without more evidence.
On one hand I have heard talk of wardens glitching their way into castles with their frozen trap thing.
On the other hand, it is possible that 3 people died inside, 1 enemy NB remained hidden and rezzed them all, and long after the defenders left they began sieging from the inside. Sounds unlikely but honestly it happens-- I've done it before-- and it's actually pretty funny so you might be surprised about the patience some people will show to pull this off.