The island's capital, Visby (pop. 20,000), the "city of roses and ruins" on the northwest coast of Gotland, is the seat of the governor of Gotland county and the see of a bishop. Within its enclosing walls it has preserved much of its medieval atmosphere and received the designation of UNESCO World Heritage Site. Of its original 17 churches,
only 10 survive as ruins and only one (St Mary's) is still used for worship. From the beginning of July to the middle of August Friedrich Mehler's mystery play (with music), "Petrus de Dacia", is performed in the ruins of St Nicholas's Church. A Medieval Week is held in August.