An acropolis was in ancient Greece a settlement, especially a citadel, built upon an area of elevated ground—frequently a hill with precipitous sides, chosen for purposes of defense. Acropolis also had a function of a religious sanctuary with sacred springs highlighting its religious significance. Acropolis became the nuclei of large cities of classical antiquity, such as ancient Athens, and for this reason they are sometimes prominent landmarks in modern cities with ancient pasts, such as modern Athens. One well-known acropolis is the Acropolis of Athens, located on a rocky outcrop above the city of Athens and containing the Parthenon.

More details: https://en.wikipedia.org/wiki/Acropolis