This remarkable 12th-century monument, the Tower of Toghrul, is located in the city of Rey in Iran. It once bore a conical roof, which is thought to have been destroyed by an earthquake. Its uses have been widely disputed. Some say it was a form of sundial, by which the time could be read according to where the sunlight fell on its exterior. Others that it was a lighthouse, guiding Silk-Road travellers on foggy nights, or, perhaps, that it served as a mausoleum. What is undeniable, however, is its structural beauty, evidenced in this colourised version of a photograph believed to have been taken by Luigi Pesce in the 1860s.