To enjoy spectacular bird's-eye views of the interior and exterior of Siena's cathedral, buy a ticket for the 'Gate of Heaven' escorted tour up, into and around the building's roof and dome. Tour groups are capped at 18 participants and depart at fixed times throughout the day – purchase your ticket from the office in the Complesso Museale di Santa Maria della Scala. Note that you'll need to arrive at the meeting point at least five minutes before your allocated tour time.
The tour is included in the OPA SI + and Acropoli + passes.