From the World Heritage inscription:
Standing in a large green expanse, Piazza del Duomo houses a group of monuments known the world over. These four masterpieces of medieval architecture – the cathedral, the baptistry, the campanile (the ‘Leaning Tower’) and the cemetery – had a great influence on monumental art in Italy from the 11th to the 14th century.
Most people have heard of the Leaning Tower of Pisa, but most people have no idea of the context of the tower. It is actually just the bell tower for the Pisa Cathedral. The entire cathedral complex is as impressive as the tower. The cathedral is a mishmash of old bricks from previous buildings and has strong Arab influences. The best part of going to Pisa, however, is watching tourists take photos of themselves trying to hold up the tower.