The so-called "Old Town" is Ostuni's citadel built on top of a hill and still fortified by the ancient walls. Ostuni is regarded as an architectural jewel, and is commonly referred to as "the White Town" ("La Città Bianca", in Italian) for its white walls and its typically white-painted architecture. A monument on its own, the town's largest buildings are the Cathedral and the Bishop's Palace, together with a number of palazzi of the aristocratic families of the region: Aurisicchio, Ayroldi, Bisantizzi, Falghieri, Ghionda, Giovine, Jurleo, Marseglia, Moro, Palmieri, Petrarolo, Siccoda, Urselli, Zaccaria.
In the surrounding countryside there are typical Pugliese "masserie", fortified large estate-farms, one of which, San Domenico, was once held by the Knights of Malta.