Heroes 5.5 WarGame Edition - Tactical Edition.

Not bad, but I think tier 3-4 is too similar and shouldn't be divided. Also, I think giving Fortress the ability to speed up tier 3 units is too much (it didn't speed up units in the standard game).
I would do it like this: Fortress - tiers 1-2, Stronghold - tiers 1-4, Castle - tiers 3-6 (if a boost for tiers 1-2 isn't needed).

What do you think of this idea?

P.S. I think the unit stats are pretty much finalized. Now we need to look at the other factions. I think we should first create pre-release building schemes for all factions, and then adjust the various upgrade options for Fortress and other buildings accordingly. And then, as you said, we can balance the growth rates.