Can you think of any examples of architecture that were built over a long period of time by many people?

Most architecture of the 20th century is designed by one or a small group of architects or starchitects. (e.g. Frank Gehry, Mies van der Rohe)

Can you think of any examples of architecture that were built over time by a large group or collective of people or an entire culture? From anywhere in the world in any time period? Especially one that was built in an unplanned way (i.e. there wasn’t one master plan behind it all)?