Heroes 5.5 WarGame Edition - Tactical Edition.

Fortress, Stronghold: +100% growth rate for tiers 1-3
Castle: +100% growth rate for tiers 4-6
Upgrades: one-time bonus equal to the current growth rate (enhanced by fortifications)
Farms: double the growth rate of corresponding creatures (militia)

Not bad, but I think tiers 3 and 4 are too similar and shouldn't be separated, and I think giving the Fortress such a large boost to tier 3 is too much (in the standard game, it didn't accelerate creature growth).
I would do it this way: Fortress - tiers 1-2, Stronghold - tiers 1-4, Castle - tiers 3-6 (if a growth bonus for tiers 1-2 isn't needed).

Regarding Upgrades, I've decided that they will be enhanced by modifications, but should the bonus equal the base value? (perhaps that's better than just 100 or 200% of the base).
Do Farms double the base growth rate or the growth rate with modifications? If it's only the base, that's fine, but if it's with the Fortress bonus, I think it's too much.