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It's practically impossible to take it by force; for example, if I place Gervul as a defender, he can hold off everyone.
But if it's a player-versus-player game, when besieging a Swamp castle, the following 6 castles are preferable:
1. Conflux
2. Dungeon
3. Tower
4. Necropolis
5. Castle
And even Inferno, with its high-speed units of the 6th and 7th ranks... especially Archdevils with retaliation, can easily defeat the Swamp faction.
Block the exit from the castle, and the advantage in ranged units will do the trick...
P.S. Only Rampart and Citadel will feel somewhat uncomfortable when besieging a Swamp castle, but they often simply outpace the Swamp faction in development, thanks to the wonderful female heroes, Dessa and Kiry.

