The map is in German, http://heroesportal.net/maps/view/4008
The objective is to capture a castle (as the game progressed, I completely forgot about the castle condition, stubbornly thinking that victory required defeating all enemies). Laughable, really...
I played as Red, with starting heroes Lord Leo (Ingem) and Sorsha, and a starting castle with the beautiful name The Castle of Love. There are three enemies - Grey (brown) with a Fortress, Blue (Inferno), and Teal (tea-colored :D) with a Stronghold. The map itself is quite easy, no special gimmicks. Red, Blue, and Grey start identically - one hero from the main castle, the second in the dungeon with a strong army (140 Champions for Sorsha, Wyvern Monarchs and Sultan Efreet for Grey and Blue).
Also, next to the main castle is a second one, on our territory there is a red tent (visited initially), on Blue's lands a blue tent (also visited), Grey has a white tent (needs to be visited). And a light blue tent on neutral lands is also visited.
The strongest opponent is Teal, with several Strongholds and the hero Vistan with stats over 90 and legions. His other heroes are not far behind. He cannot leave his island, the passage is blocked by tent guards.
Of the other two enemies, Blue was superior both in army and resources. It turned out he had 2 Grails in Inferno and Rampart (even though the scenario Grail was never dug up).
After bringing Sorsha from the dungeon to the home castle, I transferred the Champions and captured artifacts to Lord Leo. He became the key hero. Interestingly, after acquiring parts of the Cloak of the Undead King for him, Leo started raising dead, first skeletons; when I assembled the cloak during the game, he began resurrecting Liches. Atypical for a race not native to necromancy (at least I hadn't observed it before).
Blue and Grey were fighting each other extensively, while I explored the map, capturing castles and leveling up Leo with his Liches. When their number exceeded 5000, I started attacking the Azure guarding the portals and the cartographer. Simultaneously, I captured the Angelic Alliance lying next to my domains, defeating its guard of 90 Archangels.
After removing the Azure guards, I captured a couple of castles in the dungeon. Blue moved in after me, fortunately the road was already clear. By that time, Leo had learned Town Portal and began taking castles. Blue and Grey became victims of my amnesia; I didn't even need to kill them... But I defeated both and started preparing for the battle with Teal. 10,000 Liches against Vistan's legions – not a great matchup.
I went back into the dungeon hoping to increase the number of Liches; near the center, I captured a neutral Rampart and took the Wizard's Hat. Though it turned out unnecessary. Capturing a neutral Castle, I passed the guard of Black Knights and reached the Necropolis of Zul Darak. Captured it, and the game was over – Zul Darak is in your hands, you have won. :)
Here is the save from the moment just before the final capture of the Necropolis.