Not a Rise of the Republic specific issue, but it seems that barbarian deployable city siege artillery that you can place on the walls at lv. 3 and 4 cities does not work. The artillery never fires any projectiles. This can be recreated by playing a custom city assault battle as the defenders and selecting a city/faction with a barbarian map. I noticed this when attacking a barbarian city in a Hannibal at the Gates campaign when the defending city siege engines never fired at me. It looks like someone else has encountered this in a Subei campaign in the GC reported here:https://forums.totalwar.com/discussion/225608/siege-defences-not-firing#latest
I did test a custom battle defending using the Carthage city map and that worked correctly, so I am guessing it is related to the barbarian variants of the siege engines.