If they have a city, they still must protect it from Cwyld I imagine— I have not seen any kind of wards around. Or perhaps it's protection from far more mundane enemies, like the nomads.

[ They didn't greet the "nomads" with much friendliness, no? And a stationary city full of riches is always a tempting riding spot. Still, he can't help but be curious about a city underground. Looks like they will reach it should they simply follow the current. It might be uncertain if they will be let in, but so far the moss had not reacted to them in any way ]

But I imagine this answer brings more questions than answers.