Fenétre dans la coupole.

Here three things interest:

The first is the shape of the arch. That means something but I forget what.

The second is the window in the dome. What use could it have been?

The third is the blocked apex of the dome.

On the next visit I hope to get up there to check the top side and also get a shot at what that window is pointing at because it isn't there to provide a scenic view. Maybe it is there to let sunlight, moonlight or starlight in but if so why?

There is something similar built into the Panttheon at Rome. The difference is at Rome the apex is open. Might this dome at Aulnay have been too?