Arian Baptistery in Ravenna

You're standing at the Arian Baptistery, a small baptismal chapel in Ravenna. Start with its shape. The little building has eight sides, four apses and arched openings high up in the walls. It's made of brick, some of it recycled from even older buildings. Does it seem to sit a bit low? It does: the original floor is now about 2.3 metres below ground.