The strongest castle in Heroes of Might and Magic IV.

I think in any! I almost level up in all of them... it's just that what I mentioned was the last Campaign I played. Not a licensed one, the Campaign about King Arthur (Excalibur). But Bob was stronger with me because he had all the magic. And here, Tactics hindered...

Bob couldn't be stronger when all the magic skills are a disadvantage for the hero! The hero dies almost in the first round, there is no armor, and magic hits him hard. When I played through the Campaign about him and his friends, I leveled them all up to level 40, but with almost all skills, and I had a hard time with Bob, having to close him out, then resurrect him, no, it's better to have 3-4 magic skills plus Tactics or Combat

Added 2 minutes later

I haven't played many custom Campaigns.

Is Bob the mage Bohb from TGS? I actually had a hard time leveling up in his Campaign... Bohb's versatility became a difficulty for me - not only did all kinds of magic drop when leveling up in such an unpleasant way that it was even difficult to level up to Expert, but the inability to take Combat made him extremely vulnerable.

Exactly, but it's not difficult to raise his magic to Master, there are Institutes, I managed to max out all his magic skills, there are a lot of Institutes and Conservatories on the maps. But he is weak... uuuuuu