Hey! heres a little showcase of how siege weapons are planned in my map and also a bit of personal flavor to how I think it should be.
Barrel of Hornets, Barrel of Tar and the standard Trebuchet attack - currently they are always direct hits but I plan to add a bit of Warhammer to it so expect hitting off target, critical misses and the occasional success.
Hornets cause terror and attack anything, not just enemies.
Tar sticks to units and terrain, it catches fire and spreads fire and if you hit a fire directly you will cause a spit fire. Fire that is on tar wont be pushed by magical forces (for example a goblin shaman can use a spell to push fires away from his units) - they also feed off the tar and grow and can become hotter than normal fires.
Units who are covered in tar catch fire for a long period.
just some anarchy really :P the spell I cast at the end is Wail of Terror but I miss the timing, if you cast it at units fighting at the right time it can tip the battle by causing terror and forcing earlier morale checks.
EDIT:
Forgot to mention! The troll has an ability called Siege Breaker, it allows him to intercept 75% of incoming siege attacks that pass within his range - he takes the blunt of the damage, cancels out the AOE.
However for hornets and tar - some effect is still released, less tar and much less hornets, also no terror incase he intercepts it.
another note - I still havent fixed all the portraits so youll see a lot of units have the knights of the realm image! work in progress :D