Everything went super well at Pietro's appartement, my friends and I had a great time there, it was fine for 4 people. The flat is comfortable, centrally located and very clean. The bathroom is modern and the kitchen is charming inside the stone walls.
Cisternino is a really nice little town, all white with small streets and nice restaurants. It is between Ostuni and Arbellobello and really close to Grotte di Castellana, which was a very practical location to see things around. Also, we advice Osteria Bell'Italia to have diner and Pharmacy Bar to have a drink (few steps from Pietro's the flat).
I definitely recommend this stay!