Ahwaric
Shrubbery-hugger
Correct me if I'm wrong, but I thought that you could get xp and thus promotions from AirCombat. Aircraft in BtS can get promotions.
Unfortunatelly, I do not think so. I just tested it with siege and archers and ranged combat do not give any xp, even if it kills a unit (which I made possible for dwarven cannon). That is because I use ranged combat, the one used by starbases in final frontier. Air combat provides xp, but it is not exactly what I need
Also, I'm pretty sure that (unlike in Civ III) you can cam make units have attack, defense, promotions, etc, and still be capturable. The promotions don't carry over when captured, but that makes sense if it is a new crew on the old machinery.
You are RIGHT!!! Forgot about capturing animals and such. I made all siege engines capturable

Well, maybe. I still think that being able to have Dwarven cannons attack from a Peak, where it is essentially invulnerable, is a little much. Having the promotion that lets them move impassible also reduce cargo capacity by 1 would be a very simple fix.
Yeah, you are right. It should not allow to carry siege engines up there. "Can enter impassable terrain" is hard to balance anyway...
I did some tests on carring siege machines and decided I do not want it. Some reasons:
1. Siege engine can attack/bombard while loaded.
2. While loaded it can't be counter - bombarded
3. While equiped it is immune to flank attacks
4. Destruction of the carrier also auto-destructs the engine
This makes siege engines almost indestructable if mounted on a strong unit. Also, it is almost the same as adding a promotion to the melee unit.
I think some kind of spell castable by melee units that speeds siege engines would be better.
As for limiting it to dwarves - why? They have better siege equipment already. Mere warrior can pull as good if not better than dwarven champion

Also, it wasn't limited to just the Dwarven civs before, as other players could use Soldiers of Kilmorph, Paramanders, Stonewardens, and Runeskeepers.
If they have RoK as state religion. I do not think RoK should be better at siege than i.e. Order.