The Battistero Di San Giovanni is a religious building located just behind the Cathedral (Duomo) de Siena. This Battistero dates back to the 14th century and in its interior houses a beautiful baptismal font made of marble.
It is by the artist Jacobo della Quercia and it has several bronze panels that recreate the life of Saint John the Baptist. Its walls are decorated with frescos.
It's a magnificent work of art that you shouldn't miss.
I recommend you buy the "all-inclusive" ticket that lets you access the building, even the duomo. I'll explain it to you in a different place.