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

On that note, if you are interested in the complexities of building a multi-generational structure, I really recommend Kafka’s short story, The Great Wall of China. It is not about the actual historical wall, as far as I know, but it uses it as an example to make some genius insights about human society and psychology.

http://www.kafka-online.info/the-great-wall-of-china.html

One of my favorite would be https://en.wikipedia.org/wiki/Petra

Which even today looks like it would take many years to produce as is.