Unfortunately, I don't have much experience playing AS Fortress...
but I have a lot of experience playing AGAINST Fortress... and that experience is mixed (for me)... it's very difficult to defeat Fortress in a castle battle – there are swamps everywhere, and even good logistics skills don't always help (I usually ignore pathfinding with my main hero...). The creatures have such high defense that my Grand Elves barely manage to deal any damage...
The dragonfly kamikazes completely block (the Elves) and almost halve their numbers in one attack... Only one thing saved me – teleporting a LOT of dendroids to the enemy territory, supported by dragons and a few pegasi... otherwise, I wouldn't guarantee the outcome of the battle...
Fortress is a strong opponent; I don't think it's necessary to try to compensate for their weaknesses by leveling up the Witch. I think it's better to enhance their strengths, max out their defense, plus support with magic resistance, and definitely (!!) have the Tactics skill....
The strength of Fortress lies in close-quarters, prolonged combat!!!!
Gentlemen!!! (and ladies)... I cry when I play against Fortress..