Which castle is the most difficult?

It might be interesting to calculate damage, health points, and so on, but I don't want to; I just want to play.

That’s what makes the game interesting: you try to figure everything out, but it’s impossible to calculate everything perfectly, so you have to leave some things to chance. As for Dragon Utopia, you can simply visit it by defeating its guards, and then right-click to see what’s inside – the same goes for other objects, except for crypts.
/topic/6313/, check out the second post there about object values; multiply them by a monster strength coefficient (it’s better to overestimate, using 1.7 for strong monsters), and you get the guard value. The post below shows how to calculate the number of creatures in an object.
Regarding Dragon Utopia, here is a link: http://geroi-3.narod.ru/help/obyekti-shturma.htm.
And something else: physics in Heroes games http://handbookhmm.ru/tag/fizmig-2-0, animation accelerator http://heroesportal.net/other.php?storage_3/, leveling up heroes http://sergei.by/archives/AboutHeroes3/938861.htm. How to calculate damage is described in FizMig; +1 attack = +5% damage, +1 defense = -2.5% damage.
You can also watch experienced players play (it’s as interesting as watching a football match) at http://heroes3.tv/; after that, your perception of the game will change dramatically.