Ple-Sen
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Well, just so you know, Fortress isn't overpowered either. It has fast construction and Tazar, but as far as I remember, their territory consumes a lot of movement points, and this is considered one of the main drawbacks of the faction.
There is, however, a trick with a million wyverns, but in Heroes of Might and Magic there are plenty of creature banks. If not wyverns, then angels, or giants, or cyclops. This somewhat levels the playing field.