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A very interesting idea with alternative creatures, but it would be easier to implement direct creature progression.
For example, there's a peasant dwelling, and it has 5 versions.
Grades 2 and 3 could be moved to level 9 or 12 of the town and made dependent on the first-grade peasant huts.
Then, at higher levels, you could completely remove the weak peasants from the army and replace them with, for example, halberdiers.