Mortar troops are the mobile artillery of the humans. They are faster than Catapults and Ballistae, and are still able to attack the same units (for example Scorpions). They are slower than regular infantry, but have about the same range as Longbowmen. They do heavy damage, and also have the ability to take down castle walls.
Mortar troops have great destructive power and greater range than archers, but they are slower due to the heavy weight of their cannons.