Which castle is the most difficult?

But in the thread, I didn't see "The most difficult Castle in the tournament game."

Can you clarify? What exactly do you mean by "tournament game"?

I agree, on a small map, a Warrior will win, quickly overwhelming the enemy with "three Gnolls and two Goblins."

Well, the thing is... on small maps (S-M), Mages actually rule :D
And on large maps, Warriors do. (Who also become Mages during the game)

But if the map is large, with a lot of neutral creatures and objects, a hero-mage won't differ much from a hero-warrior in terms of attack and defense parameters.

Why? A Mage's development will always lean more towards strength and knowledge, simply because the probabilities are set that way.

Fast movement of Mages across the map (portal, flight, door),

Well, who said that a Warrior won't use these?

battles without losses (blindness-resurrection) allow the Mage to gather a larger army,

Who said that a Warrior won't use these?

than a Warrior (with his loss of Castles and creatures)

So, it seems that, in your opinion, a Warrior is some kind of clumsy log, wooden, without a book, who relies exclusively on close combat?? And you just build him, blow things up, and resurrect yourself? No. You are deeply mistaken.

and even if they meet in a "fair" duel without magic, the numerical superiority will be on the side of the Mage, and then it's anyone's guess who will win.

Again. Where did you get that from, that he will have a numerical superiority? In my opinion, it depends on the skill of the player.

Regarding ranged attacks: will the Castle's archers be defeated by Swamp creatures? Valeska and Vistan have a specialization in their archers, which gives the same bonus and ranged attack – let's discard these parameters, let there be 10 archers (str) against 10 lizard warriors (yaw). Archers have 2 or 3*2 damage, attack 6, defense 3, lizard warriors have 2 to 5 damage, attack 6, defense 8. It is unlikely that the lizards will constantly hit at maximum, and the archers go first...

You underestimate them. If a hero has ranged attack, and Vistan does, then they will deal a lot of damage. Of course, he also has a special ability. But even without a special ability, he is good with ranged attacks.
You need to consider everything as a whole, and not just archers with zero-level heroes. Take a Swamp hero, he levels up well in defense, improve the lizard warriors. And go to a town, say, to rest with a stack of Griffins, what will happen to Valeska's archers? They will be eaten by many. The lizard warriors will also suffer losses, but there will still be someone to shoot at!
Let's say when there are 70 of them, that's already very serious. And it is not uncommon for Vistan to defeat Valeska in ranged combat. (Of course, the heroes always have different parameters)

Regarding rapid movement – you need to constantly leave slow creatures to guard/in the Castle/with another hero – do you manage to do this every turn? ...

What's so special about that? Near the main hero, there are always weak heroes to whom the army is temporarily given for the night.

I don't have time to play against people.

Well, then what is there to argue about? It's better to convince yourself.

What I'm trying to convey to you: if you start with a Warrior, it's easier to get him to have a portal, flight, explosion, and so on. Because on the map itself, he can defeat more monsters and gather an army faster. With which he will acquire valuable items. But a Mage needs to replenish his mana, and mana wells are not on every corner. Returning to the Castle for the night, etc. And all this is wasted time, during which the Warrior will build up his advantage.

It should be noted that I am talking about playing on random maps of large size.