The Campanile de Giotto is the Bell Tower next to the Duomo in Florence, Italy. Designed in 1334 by Giotto di Bondone, it is 414 steps and 278 feet up in the air. The staircase is spiral with a hollow center so that when you reach the top you can look all the way down to the ground floor. You can see the city of Florence all around you once at the top.
I chose this photo because it has the bell in the foreground with the Duomo behind.
As an update, the wire was put up to prevent people from jumping, not to have an enclosed tower.