Santa Maria del Fiore, the main cathedral of Florence, probably is the city's most easily recognizable structure.
The big orange dome of the duomo can be seen from quite a few streets away, and as you get closer, you begin to recognize the distinctive white, green and pink marble design on the facade.
From the outside, it has to be one of the most beautiful churches in the world - at least, it's the most beautiful one I've ever seen! Construction of the duomo began in 1296 and lasted until 1436.
The duomo is huge, measuring 153 m in length and reaching a height of 90 m under the dome; it's estimated that 20,000 people can easily fit inside. In contrast to its exterior design, the interior of the duomo is surprisingly sober, but still very impressive.
My favourite features were the mosaics covering the entire floor of the cathedral, as well as the fresco painted inside the dome, which two painters, Vasari and Zuccari, took ten years to complete.