A Gothic window of the unfinished extension of the Siena Cathedral (Duomo di Siena) became a portal leading down the Via di Agnese Monna. This ambitious extension was purely politically motivated to compete with Medici Florence. Even without the black death of 1348, Siena never had the financial resource to complete it. The floor of the proposed nave on the other side of this wall now serves as the parking lot of the attached museum.
Click here to see the portal in more details.