Most of the interesting bits of Livorno are within the old Porto Mediceo part of town (laid out by Florentine architect Bernardo Buontalenti in 1567 on behalf of the Medici), still surrounded by its five-sided Fosso Reale canal. The canal is bridged in the east by an enormous vault called the Voltone, better known today as the huge rectangular Piazza della Repubblica, off the north of which you can see the Fortezza Nuova, which today is home to a popular, if somewhat downtrodden, public park (daily 10am-7pm). Just to the southwest, along the canal is Livorno's 19th-century market building, the Mercato delle Vettovaglie, still a wonderland of local flavors (closed Sun).
Via Grande leads west from Piazza della Repubblica, passing Via Madonna on the right, with its three baroque facades before spilling into Piazza Grande. The Duomo here, laid out in 1587, had to be almost entirely reconstructed after World War II bombings. The piazza used to be much longer, but a modern building has separated it from what's now Largo Municipio to the north, where you'll find the bell tower and double stairs of the 1720 Palazzo Municipale and, to the left, the 17th-century Palazzo di Camera.
Beyond is the somewhat ambitiously named Venezia Nuova district, where the 18th-century merchants' palaces are visibly crumbling, but you'll still find plenty of character along colonial Via Borra. On nearby Via San Marco, spot the facade of the former Teatro di San Marco, where the Italian Communist Party was founded in 1921 -- marked by a plaque and the red flag, still flying.
On the water is the Fortezza Vecchia, designed by Antonio da Sangallo (1521-34) and built into a 1377 Pisan fort with the stump of the 11th-century Torre Matilda poking perkily above it and hiding a Roman castrum in its bowels. The road along the harbor gives a good glimpse into the life of a still very much alive and functioning seaport, and where it hits the other end of Via Grande coming down from the Duomo is Livorno's prize art sight, the Mannerist Monumento dei Quattro Mori (1623-26). It's the masterpiece of Giambologna's student Pietro Tacca and is much more famous for the four bronze Moorish slaves lunging and twisting in their bonds than the statue of Ferdinando I above them.
South of the Old Town, Livorno's modern art collections are at the Museo Civico Giovanni Fattori. It's housed in the Villa Mimbelli, which is surrounded by a lush public park on Via San Jacopo in Acquaviva (tel. 0586-808-001; http://pegaso.comune.livorno.it/index). The villa is a trip, designed by Liberty architect Vincenzo Micheli in 1865 for a local businessman; its sumptuous restored interior contains quite an eclectic grab bag of styles. The museum is almost entirely dedicated to the only Tuscan art movement of note over the past several hundred years, the Macchiaioli, whose painters were interested in the way a viewer perceived the marks of light and color on a painting and covered canvases with their own take on French Impressionism. The museum is dedicated to one of the Macchiaioli's greatest talents and Livornese native son, Giovanni Fattori, whose work spanned the 1860s to 1908. Admission is 4€ adults, or 2.50€ for students and children. The museum is open Tuesday through Sunday from 10am to 1pm and 4 to 7pm. Take bus no. 1 or 8 here.
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