The Architect

The architect gathers scrap materials from destroyed enemy structures and builds portable artillery out of them, like trebuchets/cannons/mortars/xbows on wheels. The architect could use these weapons himself or surrounding troops could operate and/or push the weapons around for him.

How about a spell that, when placed over destroyed defences, makes those defences weaker but on your side (lightning spells can damage your defences aswell) (when the structure dies troops underneath take damage)