High above the ground on medieval cathedral building sites, men sometimes went to work by climbing inside enormous wooden wheels.

Known as treadwheel cranes, these machines were among the most important lifting devices available to medieval builders. A worker, or sometimes two, walked inside a wheel much as a hamster runs in its exercise wheel. Their weight turned an axle around which a rope was wound, providing enough mechanical advantage to hoist stone, timber, tools and other heavy materials high above the ground.

As Gothic architecture became increasingly ambitious during the Middle Ages, the ability to move heavy loads vertically became particularly valuable. Cathedral walls rose tens of meters, towers climbed higher still, and thousands of pieces of cut stone had to reach masons working on scaffolding far above street level. Large cranes could be erected within towers or on the upper levels of a building and incorporated into the construction process as the structure rose. Research into medieval tower construction suggests that large centrally positioned cranes were regularly used for this purpose.

Some remarkable survivors show just how substantial these machines were. Canterbury Cathedral still contains its original treadwheel high in the Bell Harry Tower, completed in 1498. Large enough for two men to walk inside, it lifted stone, bricks and tools during construction and continued to be used for centuries afterward.

At Salisbury Cathedral, a 14th-century treadwheel survives inside the timber structure of its famous spire. Historic England describes it as a winch for lifting heavy objects, functioning much as a crane atop a modern skyscraper does today.

Medieval cathedrals required extraordinary knowledge of geometry, masonry and structural engineering. They also depended on a considerably simpler source of power: people willing to spend the day walking in very large wooden wheels.