Pisa is known worldwide for its leaning tower. It really is a quite remarkable structure and its inclination makes it special. So when they started to built this magnificent tower without knowing about instability of the ground - they actually did to the city a big favor. Even though it costs so many troubles to keep it on the ground. But it is so famous and all over the world people wanna come and do the funny "I am holding the tower" pose. I wasn't an exception. I know, it is not very original. But I couldn't resist:).
Pisa isn't just about the Leaning Tower. Like most Italian cities there is also a Duomo (Cathedral). And it is impressive as well.