The tower began to Sink

The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-meter foundation, set in the weak, unstable subsoil, a design that was flawed from the beginning. Construction was subsequently halted for almost a century, as the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled.