Arty (including non-gunpowder) generally doesn't bounce. Some people say they have seen it on rare occasions, I don't recall any specific incidents where I saw it but that doesn't mean it doesn't happen. In Rome the onager etc missiles would bounce almost every time which is probably why people think it is commonplace in M2TW.

Also, mortar fire is extremely devastating. Only a few factions have them (namely the Italians and maybe a few others). Besides taking down any wall/tower with a few shots, they are very useful against infantry. They are very accurate, can fire over the tallest walls (or even onto units standing atop the walls) and even though they don't have explosive shells, they still have a huge area effect, killing about 8 men with every hit.