We already have this in Civ6. Unless you get nailed right at the start with the AI’s crutch of extra units, or Hammurabi is in the game and your warrios are facing Men at Arms it’s trivial to render the AI harmless with a minimal army
Just have walls and encampments in your border cities, along with a ranged unit for the encampments and a ranged or melee unit in the city centre and the AI can basically be city/ranged striked to death with minimal effort.
I'm gonna be honest. Why would I waste a district slot on encampments?
Even if I get declared war upon out of nothing (highly unlikely, I tend to have DoFs with most if not all AIs, and if not there's a good chance I'll have my suspicions about an AI already), I can build up a sufficient army to defend myself before the AI can take my city. Just gotta build a bunch of ranged units, station them in cities or on defensive terrain, and maybe a few melee units (or, if vs'ing cavalry, anti-cav) to defend everything.
I can't remember the last time I've lost a city to the AI in a war that wasn't declared in the Ancient Era (if it ever happened...), and frankly, beyond not building encampments I tend to not even build walls. Again, unless attacked.
Honestly, I build encampments in two situations:
-Because it looks nice as a defensive fortification, e.g. blocking off a mountain pass.
-Because I'm going for a conquest victory and didn't forget that you need encampments for Great Generals. (I've played conquest victories without encampments where I
did forget this...)
It happened every game for me. I would mod in maintenance changes to try and reduce the problem.
I saw Spain with a unit in every tile, and pushing land units out into the sea because there was no room left on land.
I can't imagine ever seeing a carpet only 3 tiles across.
And what about your troops?
Its just no fun managing and parking a hundred troops. Or moving a 50 ship navy.
Are you like, playing a different game?
As others said - I'd like to see some screenshots. Because I have no clue how this would happen. Booting up Civ VI right now, here's about what I usually see in a late game on Deity (taken from the last end-of-game save I have, which is like a year old or something) - note that I specifically went for the AI with the highest military score, and revealed the entire map so nothing could hide in fog: