Via Banchi di Sopra, Via Banchi di Sotto, and Via di Città all meet at the Loggia della Mercanzia, begun from Sano di Matteo's plans in 1417. Here Siena's merchants argued cases before a commercial tribunal so impartial that foreign governments came to have them settle financial differences.

From here, several tunnel-like stepped alleys lead down into Piazza del Campo (aka Il Campo), arguably the most beautiful piazza in all Italy. Crafted like a sloping scallop shell, the Campo was first laid out in the 1100s on the site of the Roman forum. The herringbone Siena brick pavement is divided by white marble lines into nine sections representing the city's medieval ruling body, the Council of Nine. The Campo's tilt, fan shape, and structure are all a calibrated part of the city's ancient water system and underground canal network. At the top of the Campo is a poor 19th-century replica of Jacopo della Quercia's 14th-century masterpiece fountain, the Fonte Gaia. Some of the very badly eroded original panels are kept in the Palazzo Pubblico .

The only surviving medieval buildings on the square are, at the top, the curving facade of the battlemented 13th-century Palazzo Sansedoni and at the fan's base, the city's focal point, the Palazzo Pubblico (1297-1310). This crenellated town hall is the city's finest Gothic palace, and the Museo Civico inside is home to Siena's best artwork . When the Black Death finally abated in 1348, the city built a loggia chapel, the Cappella della Piazza, at the left end of the palazzo's base to give thanks that at least parts of the city had been spared. Rising above it is the slender 100m (337-ft.) tall brick Torre del Mangia (1338-48), crowned with a Lippo Memmi-designed cresting in white travertine. It was the second-tallest tower in medieval Italy and was named after a slothful bell ringer nicknamed Mangiaguadagni, or "profit eater." (There's an armless statue of him in the courtyard.)

If you fancy climbing 503 steps and aren't particularly claustrophobic, the tower offers an unforgettable view across Siena's cityscape and the rolling green countryside beyond. Admission is 5€ ($5.75) with a reservation and 5.50€ ($6.30) without. You can reserve a time to climb the tower by calling tel. 0577-41-169, faxing 0577-226-265, or e-mailing moira@comune.siena.it. The tower (entrance in the courtyard) is open from November 1 to March 15 from 10am to 4pm and from March 16 to October 31 from 10am to 7pm.