Haven vs. Fortress (how to play against dwarves?!)

Option 1: Try developing Darkness + Machines + Attack.
Option 2: Try a combination of Light + Machines + Attack up to Fire Arrows, but in that case, it's better to develop Vittorio.
In short, to be sure – as soon as you find out what you have in the magic guild, develop that magic; key spells:
for Darkness magic: Berserk and Dominate.
for Light magic: Divine Retribution.
And since you'll be playing for a long time, I think you'll have time to train your troops well (if you have a lot of money, create a stack of Paladins that can defeat any Dwarf stack in one hit, and then the Resurrection Rune will be useless).
In general, Dugald or even Axel would be good heroes.
But there's also another strategic way to win: take Rutger and develop his War path, and you'll reach level 25 much faster, and accordingly, you'll conquer more money and artifacts (if, of course, you don't clearly define the boundaries of the territory that cannot be crossed until a certain time).
In any case, Haven and Necromancer should not be inferior to other castles in the late game, if not more so. :)