Siege equipment does not benefit from Blacksmith technologies, and eventually can be outranged by crossbowmen. Therefore it is wise to hire Siege Engineers! You'll need to research a rather expensive technology at the University to obtain them, but once you do, all ranged siege equipment (including War Wagons) is able to fire farther (+1 range), and their shells deal 20% more damage to buildings. And, just like the unseen people who conduct your Town Watch, the siege engineers take up no population room.
Researched at: University
Cost: 500 Food, 600 Wood
Effects: Siege Weapons +20% vs. buildings, +1 range (except rams).