I haven't taken capitals with Elves or Dwarves; I just haven't had the opportunity.
With Humans, I took the capital thanks to Assassins – poison deals damage regardless, as already mentioned. The main thing was to give everyone +30 armor potions, accuracy potions to the Assassins, and cast a protection spell on them. That's it, the capital falls.
And, in theory, with artifacts that cause petrification and paralysis (the Incubus Horn and the Soul Crystal) or the Witch's Ring, which turns enemies into Imps, you can take any capital with any army, as long as the hero hits. And the Blade and Skull of Thanatos, again, significantly speed up the process of killing due to poison.

