Why not invent some sort of citadel?
Do it kinda lika city mechanics? Iow, for ex 1500pop is needed to make a city into a citadel. Res/cropfields upped to mx lvl15. Not able to fully describe it, like i see some others do, as i just tought of it looking at the metropolis and this topic
Lvl 12 already takes 30 days (32 for iron) to pay for its own costs (no gold bonus, oases, or modifier building).
Lvl 15 takes 83 days. There are very few realistic scenarios where you actually want this for anything other than your capital.
Cities already offer a safe option for expansion. Cities produce almost double the resources of a village and they are a lot easier to protect.
Lets not further stimulate safe (boring) play.