With a little imagination you can return to 1800 when the church was still used by the nuns and you could listen to the liturgical celebrations.
Worshippers and the nuns attended mass. But there was a problem!
The nuns could not be seen by anyone because they lived in seclusion.
So how did they attend mass? Galleries, like a sort of box seat, were built with louvre blinds (grilles), behind which they could watch the celebrations without being seen.