Star Wars mod.

Perhaps, why not implement a script-based dependency where the creatures in a hero's army depend on the buildings constructed in a specific town.

Create a town-inventory, modify the town menu, and so on, so that building a structure is essentially equivalent to changing equipment, etc.

Something similar has already occurred to me, but I was thinking of implementing the upgrade system a bit differently: make creatures of one level upgradeable to another, and make the "upgrade" increase equal to 0. Then, in the castle, it would only be possible to upgrade first-level monsters. The problem was that creatures with a 0 increase are always present in a quantity of 1 on the map, which means that these are purely in-castle monsters, and you would need to introduce a lot of neutral creatures to diversify the mine guards.