Mystery hole at Moseley Dovecote

An open weekend at Moseley dovecote a couple of weeks ago and we got to learn about the importance of keeping doves to eat, to ensure a year-round supply of meat before the agrarian revolution.

Next to the dovecote the volunteers had been digging to prepare for an historical garden they're going to plant, when they found the circular foundations in this picture.

What is it? A workshop, perhaps? There's a little circular pit to one side. Only a couple of foot deep, it has a vertical pipe coming up from its base, and another emerges horizontally from its side, like a drain.