Perhaps the world's most famous tower, the Leaning Tower of Pisa is the bell tower for the adjacent to the Duomo of Pisa. The lean began not long after construction began and got worse as floors were added over the approximately 185 years of the tower's construction. As can be seen in this image, to compensate for the lean, engineers built upper floors with one side taller than the other. So not only does the structure lean it's curved.