sites without features

[Kaffa]

hi,
every day there are some novelty....I found "Jervellan Wall" and there's no explanation (!?)
Is there a guide about various sites without features (for example: "Jervellan Wall" ?
This site doesn't produces gems or pearls or other malicious trick ...why?

thanks in advance

[Micah]

It creates a fort, but not if the province already has one. Same idea with the abandoned lab site.

[Quitti]

Also Firbolg Fortress. And Crystal Citadel. And I guess there are some others.