The answer to this question is not definitive, as there are a number of buildings that have taken a long time to build for various reasons. However, some of the contenders for the title of "building that took the longest to build" include the Great Pyramid of Giza in Egypt, the Great Wall of China, and the Cathedral of Saint Basil in Moscow.
The Great Pyramid of Giza was built over a period of around 20 years using around 100,000 workers. It is thought that it was constructed using around 2.3 million blocks of limestone weighing an average of 2.5 tonnes each. The Great Wall of China was built over a much longer period â€“ construction is thought to have started in 7th century BC and continued until 1644 AD. It is estimated that around 1 million people died during its construction. The Cathedral of Saint Basil in Moscow took almost 400 years to complete, with construction starting in 1555 and finishing in 1937.