The third hero is the Engineer. The Engineer is a defensive minded hero that focuses on cheaper, better, and more durable castle defenses, and improved artillery. He has a huge, custom made musket-cannon which - while possessing medium strength - can also knock down most lesser enemies at range. His abilities range from constructing a free Catapult, to his Leadership ability which improves all other dwarfs damage, to his Hardened Walls ability, which makes it so building take no damage for a limited period of time.
His elite unit is the defensive minded Dreadnought, a heavily armored dwarf with a huge hammer that patrols the ground around your castle walls. The Dreadnought has the highest armor rating in the game, and can shrug off anything but the most powerful attacks.