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I won't try to dissuade you, I'm just confirming what you said.
Taking Swamp creatures (Barbarians and Inferno) when playing against a human player, especially on large, computer-generated maps with 200% difficulty, is SUICIDE.
There are plenty of downsides to this Castle that make it worth forgetting about:
1. The area around the Castle is full of swamps (moving quickly in the early stages of the game will be problematic).
2. The lack of a decent ranged unit in the army is also a big drawback.
3. Three-level magic (not promising).
4. Out of the two flying units, only Wyverns are reasonably good; Dragonflies are cannon fodder (combined with the lack of decent ranged units and three-level magic). How to capture enemy cities in the early stages of the game (the first two months) remains a mystery...???
Frankly, it's an unpromising and weak Castle, and you can't compensate for this with cheap units and buildings... there are too many tactical drawbacks.
2. The main ability of the Castle is defense, and it helps ranged units much less than warriors. The only ranged unit is weak, but the Fortress can easily kill enemy ranged units (often the Tactics skill is learned, and there are Snakes).
3. Three levels is, of course, a little, but this is compensated by good defense. Besides, what prevents you from taking another Castle and developing the Mage Guild in it?
4. Sieges are indeed difficult in the early stages of the game, but Dragonflies are very helpful: 1) The Fortress gets the first turn (at the beginning and in the middle of the game) in battle; 2) Dragonflies can easily block enemy ranged units.
Let's discuss, gentlemen!
My opinion: The Swamp can only compete with the other races on the later stages of the game.
Please convince me otherwise. Because I lost 200% :(