The famous tower leans precariously at a five-degree angle, giving it an offset at the top of over five metres.
Yet the 58-metre tall, eight-storey tower has managed to survive, undamaged, at least four earthquakes that have hit the region since 1280.
Construction of the tower began in 1173, and five years later it began to sink, when building had progressed to the third floor.
The cause was a flawed design – it had a foundation that was only three-metres deep set in weak, unstable subsoil.
Back then that area of Italy was very belligerent, with various local land-grabbing factions jostling for position.
Because of the battles between Pisa and nearby Genoa, Lucca and Florence, the construction of the tower was put on hold for almost a century.
Thankfully this allowed enough time for the soil to settle – had there not been that length of break, many believe the tower would have toppled over centuries ago.
When tools were picked up once more, under architect Giovanni di Simone (who had built the Camposanto Monumentale, the fourth and last building to be erected in Cathedral Square) in 1272, the engineers built upper floors with one side taller than the other, in an effort to compensate for the tilt.
Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria, and the seventh floor was not completed until 1319. Its stewardship at that point had passed to Tommaso di Andrea Pisano.
Finally, the bell-chamber was finally added in 1372.