Not sure if this has been suggested before but I think this would make unique unit infantry more viable in imperial age: The elite upgrade from the castle should also allow the unit to be created at barracks. Obviously Goths would then need a new unique tech to replace Anarchy, but I think the idea is solid.
Elite Jaguar warriors / Woad raiders / Samurai / Teutonic Knights etc. spam all coming from barracks. I honestly don’t think it would be overpowered, especially since scorpions have had such a big buff. I guess the only downside is that unique infantry units counter champions, but remember there are only a handful of civs who have unique infantry, so you would have to weigh up the civ matchup and scout their buildings.
An interesting idea, but I wouldn’t want it to be for all of them. First, some would be way overpowered if mass-produced. Second, killing castles as a way of cutting off UU production is a good thing to have as a possible strategy. From that point of view I think individual unique techs for civs that are supposed to spam their UU makes the most sense.
That said, I could see an argument for making that its own tech, something like Hoardings but just makes your UU creatable at its respective production building, and this could be given to some civs and not all of them. Then Goths and Huns could have new unique techs and still have access to this new standard one.
One solution would be for UUs requiring an existing castle to be available. Last castle lost = the unit is locked in the barracks until another castle is built.
That’s a fun idea. A bit like Feitoria/Wonder gameplay where you could have a castle hidden away or tucked into the back of your base so you can still make your UU.
I don’t like it at all. Several UUs are balanced to be produced in a limited way. Obuch cost only 20G and are absurdly strong. Imagine if the Poles could spam them.