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You could implement a system similar to the staves in the third installment of the *Heroes* series. In the third game, placing a staff creates a guardian that gains experience daily, gradually improving. It has three levels.
Capturing a garrison generates an army there. I think 285 power points per day for each slot is reasonable (if, for example, a black dragon costs 2000, and a peasant costs 75). You could also implement mines in the garrison, but they would be weaker, generating 150 power points per day. Power points are analogous to leadership points in *King's Bounty*. The first slot generates a level 1 creature, the second slot generates a level 2 creature, and so on up to level 7. If you ignore upgrades, that's fine. If you include upgrades, the question is how to implement them, especially with a system of alternate upgrades like in *Heroes 5*.