Hasbulat
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But in the second city, you can build another set of fortifications, which don't cost crystals, and that will give you +4 to creature growth :).
By the way, a question for discussion: is it worth upgrading the recruitment buildings in all cities? If you only upgrade in one, it will save significant resources, and you can recruit all troops as upgraded units if you recruit them in the city with the upgrade. On the other hand, there will be less growth, especially of the main creatures.
In the third game, you could only build one upgrade building in one castle. In the sixth game, it gives a bonus to combat units. And a very significant one! No, you can't save on them in any way. But it's a different matter to set your own priorities and think about the order in which to make upgrades - that's possible.
Some creature generators can be completely skipped. Or at least not upgraded at all, but instead develop others earlier.