MV: Threats to Nentir Vale has the Arkhosian Siege Tower in it - basically, a cross between a siege tower and a golem with a flame thrower. Haven't used one yet, but will be shortly - will have to tweak the rules, though, as it doesn't actually have a means of disgorging troops onto castle walls, instead being more of a base for archers.
Its only lower-mid paragon in level, so it may be a bit low for what you want... though looking at its stat block may provide some inspiration.
This page of Girl Genius seems relevant.