If you really want to, you can take two armies, for example, a Castle and a Fortress, the monthly growth of fully built cities, two equally strong heroes, and pit them against each other. Who would win? I would bet on the Castle.

That's more or less true; the Castle's growth is generally stronger, but the game doesn't start with fully built Castles :) What can a Castle build in the first week (let's not consider Jebus, where the Castle is the undisputed leader)? Usually 1, 2, 4, 6 in the Castle/Stronghold. Meanwhile, the Swamp easily builds 1-6 in the Castle in the first week, and often 1-7 in the Fortress/Stronghold. The Castle is only saved by the power of archers, the free upgrade for horses, and in the late game, archangels.
Plus, you can't ignore the excellent heroes of the Swamp: the armored Tazyk, the easily starting Vistan and Bron, Alkin with two important starting skills... They quite easily get Logu, Earth, which cannot be said about the Knights, whom I consider to be one of the worst warrior heroes (I'm not counting the forbidden Mullich :)).