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Heroes are upgrades, all creatures in the castle can be built, it's a commander hero rather than one of the units, it's the good old "attack-defense-spell power-knowledge" stats.
In Heroes 1 - there are no upgrades!
In Heroes 2 - there are upgrades, but not for all units...
In Heroes 3 - all creatures in the castle have upgrades.
What we see is that development is happening..
Furthermore, the fact that the hero became not only a commander but also a fighter - this is also development. The same goes for the Hero's characteristics.
You can't stand still... you'll grow moss.. :D
The world changes, and we must change too...
Respectfully.