Together with the Cathedral and Baptistery they form a cathedral complex which was was funded in XI-th century. This arrangement of group of sacral buildings was very typical for the Romanesque architecture of Italy, but only here the ingenious design and artistry of the workmanship achieved a great lightness and delicacy of early medieval structures - despite the fact some of them crooked.
The square is surrounded by also medieval wall.
.....and yes we did this funny posture ( supporting the tower) posing for the photo but we're not going to publish it ;-)
Should you like to find out more, please see the link below:
http://www.architectureppf.com/ppf/author.aspx