Nargott
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This needs to be tested. If there really is a possibility to build five-level upgrades, then it will be possible to get a boost of 3-4 times in the same tier (if needed). But each building has its own ID, so it's unlikely.
As for the tiers, the original 0-3-6-9-12-15 are already too slow, so turning them into 0-30 would make them even slower. On day 7-8 of the game, we should be able to access any top-tier technology, just like in Heroes 3, otherwise, goodbye to the game's dynamics.
Added 8 minutes later
For now, I think that the mechanic of setting the base color of the city when building a fort is the most adequate, in my opinion. Because it won't reduce the importance of the upgrades themselves (there are no selective recolorings, the base color changes for the entire city). And at the same time, it will give the opportunity to play with any of the colors as a base. Instead of 1 type of city, we will actually get 3 of its subtypes (the ability to choose one of them). At the same time, the possibility of changing this base color will remain for those tiers where an upgrade will be built.
Added 8 minutes later
For now, I think that the mechanic of setting the base color of the city when building a fort is the most adequate, in my opinion. Because it won't reduce the importance of the upgrades themselves (there are no selective recolorings, the base color changes for the entire city). And at the same time, it will give the opportunity to play with any of the colors as a base. Instead of 1 type of city, we will actually get 3 of its subtypes (the ability to choose one of them). At the same time, the possibility of changing this base color will remain for those tiers where an upgrade will be built.