This was a substantial structure that could support a lot of weight - a shed-opening party was held on the roof!
The compass shows true North and 10deg divisions. The tree is growing through a flexible collar in the roof
On the left is the top of the curved staircase leading up to the circular tower with its 3D curved-arch doorway. The ends of the straight joists forming the hyperbolic paraboloidal roof are also visible.
The stair-treads can be rotated to gain access to the top of the compost store beneath
The barbecue is on the left and the arch in the staircase wall gives access to the base of the compost store
...is under the arch at right - temporarily in service as The Bar for the Shed Opening Party