The town of Sion is dominated by the famous hills of Valeria and Tourbillon. The cathedral of Valeria is a harmonious mixture of the Romanesque and Gothic styles. By the year 1000 it became the chapter seat and accommodated a rare yet famous gem, the world's oldest organ that is still playable, originating from around 1390. The 13th-century summer residence of the bishops of Sion on Tourbillon was destroyed by fire in 1788 and is now a ruin. The All Saints Chapel, a small Romanesque-Gothic building from the 14th century, is located between the two hills, and below the Valeria is the medieval Old Town with the Witches Tower, the last preserved tower that was part of the old town fortifications.