Igr_ok
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Interesting scenario, though quite challenging.
Spells in the Castle: mostly Dark magic. Always take Berserk and Subdue on levels 4-5, so Dark magic is a good choice for heroes.
For Alexander, focus on Logistics and Machines early on.
For Annabelle, I focused on Dark and Sorcery.
Tips for the quests.
Alexander can reach the Castle in two ways. Besides the obvious route through the Tomb of the Ancient King, there's also a path through the desert. You need to take a ship and land on the other side near the Mage. He gives the quest Riddle of the Pyramid. There are also 3 refugee camps with Knights and Paladins, and more Paladins will join. And as a bonus, there's a Fort on a hill :) Inside the pyramid is an evil Necromancer who will lower morale to -8 with a single spell. But his army is small, and you can defeat him with a single ballista. As a reward, you get a Portal scroll to the city and artifacts.
In the desert, you can also take another quest from the Orcs. But to complete it, you'll need the green ring from the Necromancer's set, which can be found in the tomb. You can go to the tomb for the ring first, then sail to the desert, but I'm not sure if you'll have time to return. After completing the quest, Shamans, Executioners, and female Spellcasters (Orc archers who shoot poison clouds, like Liches) will join you.
After repelling the first attack of the Forsaken Bride, you need to quickly complete 2 quests. To defeat the invincible ghosts, you need to visit the dilapidated galley on the river. To win, you need to kill the ghosts simultaneously. Stone spikes are perfect for this. You can always get them at the exit of the Dungeon. A simple tactic: 3-4 monks in the corner, and the ghosts will line up like crosses) You get an artifact that will help you fight them. You need to equip it. Then immediately sail to the left into the whirlpool. Talk to the mermaid, kill the dragons, and take the hammer.
A Few Reliable Gnomes
Take the quest only when you're ready to complete it, because these gnomes must survive. After freeing the mine, you need to place the gnomes inside. Then click on the mine again and give them the hammer. After that, the passage to the prison will open.
In the prison, free Talanar. With his True Luck perk, he's our champion in battles where magic is prohibited. Level him up on peasants. It's very desirable to get Attack with Ranged, BB, and Retribution. It would also be good to get Storm Wind in Light.
Overcoming the Seraphs. Take Talanar, give him Master Archers, Crossbowmen, two Archangels with 1 stack each, and the rest as single units to protect the archers. The Archangels resurrect each other, and the archers do all the work. You need to kill the Seraphs three times, and their number will increase. In the 3rd time, I had almost 1000.
A long quest to find the mermaid's sister. Visit the tavern (near the stables) with only 4 crossbowmen. Get to the prison, escape, return to the mermaid, get ice elementals. Break through the Dark Elf garrison in the Dungeon, use the elementals to overcome the fire barrier, and get the ring of speed.
When you get the Elven Bow, you need to answer 7 questions. The answers are in the tavern of the Castle, with the innkeeper. There are 2 mistakes in them. Not 248, but 284. Answer the question about Popov incorrectly, because the author confused OK and Cancel (@C Remix).
You can go and attack Irina.
The garrison in front of Irina's Castle prevents the use of magic (except for resurrection in Archangels), so you need a lot of melee units. 200 Paladins were enough for me. During the assault on the garrison, there was a bug. Instead of 100 champions, there were 300.
P.S. Thanks to Remix for the video that inspired me to play this scenario :)