This unusual structure is the King's Ice House. It is in effect, a roof over a foundation and dry well in which they stored ice cut from local lakes in the winter. As can be imagined, ice would have been highly coveted and would not have been available to the general population. The Fleur-de-lis decorating the peak of the ice house rood was a sign that this was the King's property. It can also be seen in other photos in this series including the King's Bastion. See the whole set on black: flickr.com/photos/whitebeard/sets/72157613402503136/show/