Officially called the Basilica of St. Stephen in the Round, Santo Stefano Rotondo gets its more commonly-known nickname from its shape – it's one of the world's oldest and largest circular churches.
The church was built in the 5th century, and although it was altered in the 6th and 7th centuries and then quite drastically in the 15th century, the central part of the church remains the original 5th century design.
In addition to the church's shape, the other main attraction are the gruesome frescoes that line the outer wall. There are 34 scenes of martyrdom depicted, painted in the 16th century, each with a brief explanation of who was martyred and who gave the order.
Santo Stefano Rotondo is open daily from 10am to 1pm year-round, and then 2 to 5pm during the winter and 3 to 6pm during the summer. There is no admission fee, but donations are always greatly appreciated.