Tactics for the Swamp (Fortress).

The troops are made of cardboard, and there are no special abilities.

Wow, they are really made of cardboard. And they have no special abilities at all. Playing against the AI on the Swamp map is a real pleasure. Level 1 is not particularly noteworthy, but although it's slow, it only yields to Pokémon and Centaurs in terms of health. Level 2 is a very strong ranged unit. Thanks to its health, it survives and accumulates experience. Level 3 is the Snake. There's nothing to say about it. Until the AI has level 7 creatures, the Snake will allow your hero to cast spells first. The Vasili are mediocre, inferior to almost everyone. But this is perhaps the only weak unit. Next come the Gorgons, which easily defeat level 7 creatures (even if they are Azure Dragons) and also destroy high-level enemy stacks. Then come the Wyverns, which are not very strong, but they can fly and poison a stack with a fairly good chance, which is nice. Finally, the Hydras, which are simply excellent if you have an Expert Teleportation spell, which is quite common in Swamp castles.
Although the Swamp faction doesn't have the highest total hit points, the heroes of the Swamp have rapidly increasing defense. Plus, the city is very difficult to attack due to the Swamp abilities. And the Gorgons work perfectly against the enemy's "excess" hit points, leveling out the final difference.
Plus, the fast construction of Wyverns allows for a quick start. Which is important in any game.
In my opinion, the main weakness of the Swamp is the lack of any anti-magic properties. All units are vulnerable to any spells of any level.