Plovdiv is Bulgaria's second-largest city. Plovdiv is one of its most attractive and vibrant centers, situated in the Upper Thracian valley, along the two banks of Maritza, scattered upon six unique syenite hills. Certainly, there's plenty to see: the old town embodies Plovdiv's long and varied history - Thracian fortifications overlaid with Byzantine walls, and by great timber-framed mansions erected during the Bulgarian renaissance, looking down on the Ottoman mosques and artisans' dwellings of the lower town. But Plovdiv isn't merely a parade of antiquities: the city's arts festivals and trade fairs rival Sofia's in number, and its restaurants and hotels compare favourably with those of the capital. The Roman Plovdiv revolves around the ruins of a Roman stadium and forum, the Turkish Baths, the 15 century Imaret dzhamiya and the impressive Dzhumaya dzhamiya or "Friday mosque" with its diamond-patterned minaret and lead-sheathed domes. The old Plovdiv, most of which is designated as an Architectural-Historical Reserve, is remarkable with its cobbled, hilly streets, churches and colourful orieled National Revival-style houses that are Plovdiv's specialty. A splendid landmark and a venue for concerts and plays, is the Roman theatre, whose imposing ruins are practically the only remains of an acropolis which the Romans built when they made the town a provincial capital during the 2nd century.