See the Acropolis in Athens

Acropolis means "highest city" in Greek, literally city on the extremity and is usually translated into English as Citadel (akros, akron, edge, extremity + polis, city, pl. acropoleis). For purposes of defense, early people naturally chose elevated ground to build a new settlement, frequently a hill with precipitous sides. In many parts of the world, these early citadels became the nuclei of large cities, which grew up on the surrounding lower ground, such as modern Rome.