Actually, for Tier 7, we have:
1) building a base = 100% arrival rate (+1 dragon) and 100% growth rate (+1 dragon per week)
2) building an upgrade = +100% arrival rate (+1 dragon) and the ability to change the upgrade
it would be logical to use the same approach for all other tiers (according to points 1-2). That is, the question boils down to how exactly forts should affect the arrival and growth rates, and for which tiers.
Regarding point 3:
1. The size of the bonus. There are two options: +50% growth rate and +100% growth rate for building each fort.
2. The area of influence of each fort. The following options have been listed: single-level - all for tiers 1-3, and three-level - for tiers 1-2, 1-4, and 1-6.
3. Should forts affect the one-time arrival rate. There are 2 sub-questions here: should they affect the arrival rate from basic dwellings, and should they affect the arrival rate from upgrades.