The map is unreal, generated randomly.
Many castles.
Lots of gold.
Humans versus Necromancer.
Naturally, there will be enough money to convert from monks to knights.
And the Necromancer will burn all his troops into his own.
So, who will come out stronger?

Added 4 minutes ago

DagonetEspecially good is the "Sharp Mind" skill in the logistics branch, and humans don't need education to get it.

Why is Sharp Mind so good?

If the castles are of different races, this is bad for Haven. The Necromancer doesn't care what castle, he recruits troops from any castle and turns them into undead. But the Necromancer's attack and defense grow little with level, so his army, although unrealistically large, will not be very strong in terms of damage and defense. It is also difficult for the Necromancer to get logistics, it is easier for Haven, and the further the hero goes and the more artifacts he captures, the greater the chance of victory. Good artifacts are the key to victory for any castle!
Since there will be a lot of gold, Axel is the most suitable hero for Haven, especially for a beginner. There are many cities, which means there will be a lot of gold. In one week, convert 6 monks into 6 knights and 14 spearmen into monks, and in the next week, do the same, and you will have 20 monks - convert them all into knights. With "Experienced Commander" and "Skilled Counterattack," 20 knights cost approximately 26,500 gold, without skills, 75,000 gold, you are unlikely to earn that much in a week.
So, what else... Light magic is a must, defense is a must, leadership is a must. The other two skill slots are up to you. On huge maps, the advantage lies in the "Logistics" and "Education" skills.

Regarding "Sharp Mind." It gives the hero an advantage by allowing him to go first at the beginning of the battle. A hero with an initiative of 10 means that his turn will begin approximately in the middle of the round, but with Sharp Mind, he will go almost at the beginning. And this is a big advantage. For example, the hero goes faster at the beginning, casts a general speed boost or a general devastating strike on the troops, etc., and your troops immediately become significantly stronger.
Also, take "Inspiration" and "Empathy" in the leadership branch. With Empathy, the hero will go more often (especially relevant in battles with Necromancers), and "Inspiration" will help speed up the knights' turn. The main thing is to strike from a distance, the further away, the more damage the knights do. If the Necromancer enchants your cavalry, remove the enchantment or, at worst, cast antimagic on them...
Oh, it takes a long time to explain everything. Maybe other forum members will explain the battle mechanics better.