Florence was built on the site of an Etruscan settlement and the subsequent Roman colony of Florentia (founded in 59 BC). This Tuscan city became, during the early Medici period (between the 15th and 16th centuries), a symbol of the Renaissance revolution, reaching extraordinary levels of economic and cultural development. The current historic center extends over 532 hectares and is bounded by what remains of the 14th-century city walls. These walls are characterized by gates, towers, and the two remaining Medici fortresses: that of San Giovanni Battista, to the north, known as "da Basso", and the Forte di San Giorgio a Belvedere located among the hills of the southern slope. The Arno River flows from east to west, crossing the city, and a series of bridges, including Ponte Vecchio and Ponte Santa Trinita, connect its two banks. Seven hundred years of extraordinary cultural and artistic flowering are clearly visible in the 14th-century cathedral of Santa Maria del Fiore, in the Church of Santa Croce, in Palazzo Vecchio, in the Uffizi and in Palazzo Pitti. The city's history is also evident in the works of great masters such as Giotto, Brunelleschi, Botticelli and Michelangelo.