The most important are clustered around three adjoining the triangular Piazza della Ceterna named after the city 's 13th-century travertine well , surrounded by elegant palaces ; the Piazza del Duomo , grouping church and town hall as the center of civic and religious power ; and the Piazza delle Erbe ? ­ market place with twin Salvucci towers .
The Salvucci towers are open to visitors .