Here's a brief overview of my Walkthrough:
Day 1
Gathering resources, buying 8 heroes, upgrading Luna to break through the central guard, preparing a diplomat to join forces on Day 2. Also, heroes who will capture the Swamp Castle and free Rasus on Day 2 are stationed near the horses. I didn't summon any boats; I didn't plan to fish anything out of the sea.
Day 2
I recruit Griffins, Ballistae, and Gorgons, and capture an Angel in a hidden location. Then, Luna defeats the horses, Gorgons, and Manticores. Interestingly, I captured the Swamp Castle because if you don't, you get 396 points for completing the map, instead of 398! As I mentioned earlier, I had to replay the game because of this. I also want to add that capturing the Necropolis in addition to the Swamp Castle doesn't give any extra points, so there was no point in going there. Besides, the Black Knights are on cursed ground, and I didn't really figure out how to break through them, so I went for the Swamp Castle. Then, I freed Rasus, took the Utopian, collected some gold, and had the final battle.
I didn't use the external market at all.
As for something that might be interesting for beginners, I'll just add that in the battle between Luna and the horses, Sir Radost had some kind of resource advantage according to the descriptions, despite having good artifacts. However, my Luna defeats them without artifacts and, interestingly, uses only two walls of fire. Luna vs. horses - video

Also, here are my turn-by-turn guides