In the 14th century the Sienese began an enormous new cathedral, of which the existing Duomo was meant to be only the transept. The Black Death of 1348 and structural problems killed the project; what stands today is the unfinished façade and right aisle.
A narrow staircase from the Museo dell'Opera leads to a stone gallery along the top of the abandoned façade, with views across the Campo and the Tuscan hills. Access is included with the Opa Si Pass. Not for those who dislike narrow spiral staircases or open heights.