Kobal,

Especially if you want to keep the walls as intact as possible, you would want to use a trebuche. It keeps hitting the same spot over an over, thus reducing the total amount of damage required while putting one hole in the wall. That would make it much easier to fix.

DesertEagle

Yes, the amount of energy transmitted compared to total potential is reduced a little by the downward angle. On the flip side a trebuche can generally throw a much heavier projectile than other siege weapons. Cannons had a hard time getting up over the 200-300 lbs weight limit due to pressure restrictions on the barrels. Catapults and Balista had a lower limit due to their method of manufacture. Trebuche were able to throw projectiles over a ton accurately. As such, even if it only delivered half as much energy per pound of projectile it would still be far more effective than other weapons.

The down side to real world trebuches is that they were not mobile. Unlike gunpowder or tortion powered siege weapons, trebuche relied on very heavy counterweights. Some of the counter weights were over 20 tons. As such the real heavy duty trebuche were not only built in place but aimed at a specific target when they were assembled. When ranging the weapon, projectiles of a known weight were used and the mass of the counter weight was adjusted based on where the projectiles landed. This trick also allowed the trebuche to hit at the top of the wall to begin with and work its way down, simply by removing a few of the stones from the counterweight.