Except COE won’t involve spawning. It will involve a persistent world with persistent and individual NPCs. PCs persist as OPC when offline.
We can have high security zones, is we build them (a cost) and staff them (a continuing cost). Historically many great fortresses fell because they could not be adequately manned. That should happen here, too. Secure places will have to be built and defended efficiently. Secure places will also have relatively high taxation — but the economic security and concentration should still make them preferable to unsecure places for crafting and economic activities.
Finally, given the game dynamics described so far, an efficiently defended space should be able to hold out long enough for a king to send a relief force (just as happened historically), and that is a new dynamic that should work. (It is also a good reason to work with, rather than against, the king and nobles, unless they are tyrannical.). Even hordes should rampage through less defended areas and against more disorganized kingdoms.