Nestle
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In my opinion, the mission isn't difficult, although I'm playing on normal difficulty. I played it like this: first, I captured George's Castle (right at the end of the first week, his army was "small"). Then, I followed the path to the northwest – there was a neutral Castle, and I captured it at the beginning of the second week. I started developing both Castles – there was a shortage of crystals, but it didn't matter; by the end of the game, I had so many resources that I didn't know what to do with them.
Then, "Rosalia" came, and I lost one of my Castles because I didn't manage to reach it in time. After that, I retook the Castle with minor losses, waited for reinforcements, and headed towards the bridges. I went along the one on the left and captured the Castle of one of the Barons – a lady (I don't remember her name, but it was something like Irida). I started developing this Castle as well. After that, no one bothered me anymore. I developed three Castles and went to the second Baron, who was located on the bridge to the north (his name starts with the letter G). I captured his Castle. And if you go north from Irida's Castle through the obelisks, you'll find the Castle of their helper – Cordelia, another formidable lady. Igor's Castle (the only name I remember :) is located in the upper right corner of the map, but I stormed it last because he had a strong army. I gathered reinforcements for about two weeks from all four Castles – this added to my already considerable army. After defeating the three Barons, I went to find the artifact as part of the quest (it's located not far from Cordelia's Castle – southeast of it, and it's guarded by angels; I had 90 of them). Then, I went through the portal to the Dungeon, defeated all the elemental representatives, reached Kirill, talked to him, and killed him. After that, a portal opened for me, and I went through it and found myself in the Dungeon (the exit from it is located near the first Castle that I captured from George). I wandered around the Dungeon, subdued two necromantic heroes and George along with them. He had a "strong" army.
In conclusion, I want to say that it's important to develop portals so that you can quickly move around, and the "Guardian Angel" building is very useful for resurrecting riders and seraphs. It's a very convenient thing.