The monumental stone bridge over the Sieve at the western edge of the old town was built in 1555 on the orders of Cosimo I de' Medici, replacing an older crossing repeatedly destroyed by floods. It carries the road that gave the town its name — Pontassieve, "bridge over the Sieve" — and its three pointed arches still take road traffic five centuries later.
It's a short walk from Porta Fiorentina down to the riverbank, and you can drop below the bridge on either side for a clear view of the masonry. The Sieve meets the Arno just downstream; this is the last bridge before the confluence.