I think this is a great sub, though I think the entire handling of insanity is distracting and perhaps is fodder for another sub. Most systems have some rules regarding insanity. Simply indicating that those encased are subject to insanity over time would have been fine for me.
Regarding the physics being non-feasible, all it takes is to say the cement is indeed magic and forms a paper-thin coat that still happens to be as strong as concrete.
Or, the super-strong coating is created _by_ magic but is not in itself magic.
For all it matters, I'd have the coating be magic and be done with it.
I have never been a fan of expecting Fantasy subs to ascribe to closely to real world physics. Go to Comment