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The problem is that the fort on the hill is not that common, and taking new units only to later be unable to replenish troops from the Castle is inconvenient (I think everyone understands this).
And yet, if you want, you can find this fort – it is present in almost all scenarios, Campaigns, Multiplayer maps included in the collection, and maps created using the GSC. In addition, the creatures upgraded in the fort are significantly stronger than their counterparts (alts and upgrades) by about 10-15%. So, there is a point to this upgrade.
You can find out more about the new, improved creatures in this video:
https://www.youtube.com/watch?v=kjvCUBkIgm0
Is it possible to give some building the function of a fort on the hill, so that it can only be built after all housing upgrades (so that it is fair)? :)
For now, it is not possible to implement this due to the lack of the game's source code