The Porta San Niccolo (1324) stands in splendid isolation on the south side of the river, a poignant reminder of the magnificent set of medieval walls, which once engirdled the city. Thirty-five metres (115 ft) high, it is the only gate to retain its original height.
The inside of the gate is decorated with a badly-faded fresco (15th century) depicting the Virgin and Child flanked by St John the Baptist and St Nicholas.
The Porta San Niccolò was part of a set of defensive stone walls which stretched for five miles (8 kilometres) around the city. The walls were built between 1284 and 1333 and stood for over five hundred years. However, in 1865 Florence became (albeit for only six years) the capital of Italy and the medieval walls were (mostly) demolished as part of a programme of urban redevelopment.
The two-metre-thick walls originally stood 12 metres (40 feet) high and were reinforced by seventy-three towers, each of which rose to a height of 23 metres (75 feet). They were punctuated by a series of 15 gates. The original wooden doors are still hanging in the Porta Romana and the Porta San Frediano.
Over the course of its history a grand total of six sets of walls have been built in Florence. The first four sets of walls were confined to the area north of the river. It wasn't until the 12th century (1173-75) that part of the Oltrarno, the name given to the area south of the river, was included in the fifth circuit of walls.
The whole of the Oltrarno was finally enclosed within the sixth circuit of walls, long stretches of which have survived.