DarrkPhoenix: In Disciples 1 taking out a capital city garrison is incredibly difficult, and I only managed to do it on the final Empire mission. Aside from naturally having my disciple and all units max level, I used spells to max out armor before the fight, then used the mass-healing unit to heal up the damage the guardian did each turn. Even with this it still required me attacking twice in a turn because the first fight ended in a draw due to taking so long. I can't imagine doing it with anything other than the Empire due to the necessity of mass healing during the fight.
In Disciples 2 it's much easier to take out capitals, since your units can gain levels pretty much indefinitely to improve their stats (in D1 once they reached their top upgrade that was it), which allows you to create some insanely powerful units. In the final Clans mission of the expansion your goal is actually to destroy the undead capital, and just before you attack it they actually throw a custom boss enemy at you since by that point the capital guardian itself isn't all that much of a challenge.
I thought it might be possible in D1. I was thinking along the lines that it would take a series of attacks from multiple hero groups upgraded at least to carry a full party. Never imagined it could be done with just 1 party!