[Someone]

The Great Wall is impressive, but wasn’t fully built as one project. It was patched together from multiple parts (with some major upgrade/renovation projects)

https://en.wikipedia.org/wiki/Great_Wall_of_China:

The Great Wall of China (traditional Chinese: 萬里長城; simplified Chinese: 万里长城; pinyin: Wànlǐ Chángchéng) is a series of fortifications that were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups from the Eurasian Steppe