The Tower leans because it was built upon soft, unstable soil that provided a poor foundation. Construction began in 1173 on land that had been an ancient harbor, leaving deep layers of clay, sand, and seashells that shifted over time as weight was added. Builders halted construction multiple times as the structure sank and tilted, attempting to allow the soil to compress and add supports, but lean increased as additional levels caused more foundation instability. The tower’s famous lean is due to its flawed design and footing combined with the fundamentally incompatible terrain.