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Mission 3 is generally a pleasant walk and an opportunity to level up a couple of heroes, but there are some issues with mission 4.
As mentioned above, having a cartographer's hut is very helpful, as you can simultaneously manage your affairs and explore the geographical features of the Necromancer and Orc territories. It's worth the 4,000 gold. Because, although it's not explicitly stated in the mission description, it turns out that if the Orcs take even one of your cities, it's game over. Therefore, after clearing the Necromancer lands, when you prepare to march on the Orcs, you need to clearly see where your forces should block the passages to your Necropolises. If you simply rush to take the Orcs' castles, the AI will send some unit into your lands while your army is distracted. And they seem to have good logistics, plus Argath the Tracker is a relatively high-level unit that you won't be able to catch.
There is a prison on the map with the hero Kifrah, level 1. I don't know why it's there at all, or whether it's necessary to free her: again, it's not mentioned in the mission description. I rescued her, but with Aeron at level 29 and the secondary hero Lucretia at level 25 (I used her specifically against the Orcs to eliminate Argath or Tilshek), a level 1 hero is clearly unnecessary.
All the Necromancer castles are taken quite easily; essentially, they only have one hero, Ravenna, who will retreat to the field when in danger. After that, there will be no more problems.
The Orcs will NOT enter your lands until you cross any of the empty garrisons on the border. Therefore, if you enjoy long games, you can safely recover and develop after defeating the Necromancers; the Barbarians will not attack on their own.
Southeast of the starting castle, there is a Dungeon. Again, there are no mandatory requirements related to it. But when you enter, there will be a message about traps and a new task will appear, so I personally thought, "forget it," and replayed that part without entering the Dungeon, since it's not required. I thought that enemies might emerge from there and attack my rear, but no, there is no portal on that side.
Between the eastern and central Necromancer cities, there is a two-way portal. In my case, there were about 7 Archangels, level 75, on one side, and various dragons on the other. I recommend going through this portal, as there is a very good set of artifacts there. You can level up a supporting Necromancer hero on the Archangels, and then transfer all the artifacts to Aeron for later use. By the way, it's a pity that the artifacts cannot be carried over to subsequent missions.