This is Giotto's Campanile (bell tower). From this angle it appears as if it is adjoined to the cathedral behind it, but it isn't; it is freestanding. The tower was designed by Giotto after his appointment as Master of the Works Of the Cathedral in 1334. Unfortunately he was in his late 60s at the time and died only three years later. As a result, he never saw the completion of his dream. The tower was completed in 1359 after some interruptions such as The Black Death.
The tower is finished in marble in white (Carrara), green (Prato) and red (Siena) colours.
Naturally, this represents yet another missed opportunity for in the course of researching the tower after the event, I found that you can climb to the top (414 steps) and get an utterly awesome view over the city.
Oh dear. Maybe next time.
The dome of the cathedral (duomo) can be seen behind.