Pisa is famous around the world for its leaning tower but there are many other interesting sights to see in the city itself and it is well worth a full days' visit.
The Piazza dei Miracoli (the Square of Miracles) includes the famous leaning tower, or campanile, and is situated behind the Cathedral itself. Since restoration work carried out in the 1990s the tower doesn't lean so far these days (around 4 degrees as opposed to 5.5 degrees prior to the restoration). It is possible to purchase a ticket to climb the tower and enjoy the amazing views from the top, but make sure you get there early as there are usually large numbers of visitors and there is a phased entry every half hour.
The tower itself was constructed over a long period - almost 200 years, beginning in 1173, and began to sink into the unstable ground due to inadequate foundations. Ensuing battles prevented further construction for a number of years, but this allowed the subsoil to settle somewhat, preventing the tower from falling over.
Three more phases of construction ensued, the first beginning in 1272, but a further battle caused a halt in 1284. In 1319 the tower was extended to the seventh floor and then the bell tower itself built in 1372.
Restoration work began in 1990 and there is still work being performed on the tower today.
[Source: Wikipedia]