downgrading walls

[France]

im wanting to turn a city into a castle but it already has stone walls. iirc i remember seeing a city downgrade to a large town over a couple of years. idk if it was because it was constantly being sieged or if it was because the population kept decreasing or if i remember wrong. but is this possible?

[Caesar Clivus]

No it's not. Population might decrease, but the level of the walls wont.

[bɑne]

Afaik you can mod the castle <-> city conversion into the game for higher levels, try searching this forum or the mod workshop.