I had two goes at this and I suspect I might have figured out what the problem is. The first time I built stone walls with villagers and then added stone towers with infantry (enough that it should have unlocked). No dice. I tried again and only built towers on walls that had been built by infantry (also significantly more than necessary) and this time it unlocked. Perhaps it doesn’t count the structures if they’re added to walls not built by infantry or the amount of stone is incorrect.
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that’s an incredibly niche bug you have found lol
if it is so.