The Faery Rings can be upgraded by Faery Rings Upgrade Kit item. To upgrade the Faery Rings, use the Faery Rings Upgrade item on the Faery Rings that is already situated in your city. Its upgraded version will then be placed directly on the same spot in your city and the production timer will not be reset. Note that the upgraded building will be from your current era, even if the old one was from one of the earlier ages.
Faery Rings will produce the following effects. Base effects will always be produced. Bonus effects are determined by how many other unique Celtic Forest Set buildings it is connected to.