In 1172, Donna Berta di Bernardo donated sixty silver coins to the local cathedral for the purchase of stones to be used in the base of a new bell tower.
The next year, construction on the tower began and almost immediately there were problems. The tower site sat on soft ground composed mostly of clay, fine sand, and shells (the city sits between the Arno and the Serchio rivers). By the time the builders finished the second floor in 1178, the tower was beginning to lean.
Soon, construction was halted for almost 100 years, as Pisa fought wars against Genoa, Lucca and Florence. This may have been a lucky break, as modern analysis says that the tower definitely would have toppled had construction continued without time for the soil to settle.
When work on the tower started again, it was leaning 0.2 degrees north of vertical, but by the time the workers started on the seventh floor, the tower was tilting one degree south. Around this time, construction stopped again for unknown reasons.