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I agree. They're all so nasty and green :) And besides, their good defense doesn't help them much. The only decent unit they have is the Gorgon; its high damage and deadly gaze do the trick...
Otherwise, Barbarians will defeat the Swamp faction with a 70-80% probability (no offense, Swamp faction, but you're just not that good...).
Swamp is a great faction, and who said that defense doesn't help them take cities? Which Barbarian can withstand three shots from the towers? Swamp wins through attrition, and, thanks to Alan, with water magic. Teleport appears more often than you think, Celt; you just might not have played Fortress seriously.
Leneia
Why would a faction always defeat the Swamp faction? Humans are a pretty average faction; with enough effort, you can defeat them with any faction.