There are two important point:
The achievement description: "Have full guard coverage in a town with a populations of 100".

First: only men are counted, not woman! I.e. you need at least 200 in population:
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Then check that all buildings protected via menu:
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Second: achievement is buggy! If you not receive it, just destroy guard tower and build again!

You can easy get achievement in endless mode:
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Discussion: https://gtm.steamproxy.vip/app/938380/discussions/1/3570700856117452407/