At 9 a.m. on March 17, 1989, a 900 years old rectangular masonry tower collapsed without warning in Pavia, killing four persons, damaging the adjacent 15th century Duomo (cathedral) and destroying adjacent shops and dwellings. The paper describes preliminary attempt to understand the possible causes of the collapse. Initial results of analysis indicate that the form of masonry construction used in the tower, and the influence of a stairway that spiralled up around the tower within the thickness of the walls may have been significant.