Challenges of a Trebuchet

Like catapults, there existed basically two kinds of trebuchets.

There was the much simpler torsion trebuchet which was basically a larger hand drawn version of the simple catapult and the counterweight trebuchet.

In the case of the torsion trebuchet, historically that was not too effective as siege engines, rather, most accounts describe them as light artillery weapons, being used as efficient anti-personnel weapons in supportive positions alongside archers and slingers, while actual penetration of defenses was the result of mining or siege towers. 

For instance, at the Siege of Kamacha in 766, Byzantine defenders used wooden cover to protect themselves from the enemy artillery while inflicting casualties with their own stone throwers