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Now I just rediscovered the siege rules in the 6th edition BRB and integrated it into a campaign me and my friends are playing, i'm playing as VC, and there are High Elves, Warriors of Chaos, and O&G playing. All armies can get dog of war units (mainly for use of cannons) but which army do you think is well suited for those siege rules, and I mean out of all armies, not just the ones I mentioned.
Assuming some artillery from dogs of war, I think undead could siege a castle pretty well. The ethereal units can just walk through the walls, or if they get on the ramparts striking last won't matter since most units wouldn't be able to damage them. Their core units should never go up the wall though since there is not static combat res, but the grave guard would be pretty nasty assaulters. That and the whole army being unbreakable and ressurectable means that the enemy shooting won't take dish out many casualties.
But dwarfs and empire have all those lovely cannons...
Most Notable Achievements:
Killed a Revenant Titan in one shot.
Killed 900 pts worth of Blood Angels in 2 turns while only loosing 4 pts.
From what I heard, Undead really suffer in sieges, especially in Skirmish games (yes, I wondered why someone was playing Sieges with the Skirmish rules too). Although I think you've hit the nail on the head with Dwarfs and Empire. Lots and lots of Artillery. I'd have to say Dwarfs though, since they're more survivable, and have some nifty warmachines for the job. Just my thoughts.