Demon invasion.

I did the same in the second game. I, of course, leveled up Nimue. In the cities, I built gold production first, and then a building that provides +10% to the gate, as quickly as possible, regardless of the losses. I then created a one-way portal near the Castle that leads to the lower right Castle – there's easier prey for leveling up, plus a couple of dwellings. Before the first enemy arrives, the main goal is to build the Castle and upgrade it to levels 1-4, and also upgrade the guild to at least level 4 in the first and lower right cities. You can ignore the other cities and upgrades. I took heroes to level 20+, with stats like logs/darkness/luck/attack/defense. On the previous page, I posted a screenshot of a battle with the first enemy after such preparation - https://imgur.com/tXdZS4Y. After that, there are no problems.

And in Heroes of Might and Magic 3, there's a feature with the heroes. Near the top Castle, there's a two-way portal – if you enter it, the enemies behind Sigrid won't follow, and you can stay behind it, calling in reinforcements from the cities above, and at the beginning of the week, intercept the enemy's Castle, take the reinforcements, and then retreat back into the portal. This is how I accumulated troops, and then easily defeated both demons in the Castle on the Darkness map. Here's a battle with Grok that I found -