I've already asked one question that had its answer printed very clearly in the rules (re: Garrison Tokens)—I hope I'm not doing it again!
I know that Siege Engines are destroyed when forced to retreat, and I very vaguely recall reading on a forum somewhere that they can't be chosen as casualties for House Card sword icons. But I'm unable to find this second part in the rules, and the sequence of events for retreating doesn't appear to exclude it (i.e. you choose casualties before retreating, and thus before Siege Engines are automatically destroyed).
Thanks! (And my apologies if I've missed a "Siege Engines & Casualties" headline in the rulebook!)