Why is Rome often called the city of seven hills?

The phrase "city of seven hills" (septem Colles) is often used to refer to Rome. The reason is that Rome was originally built on seven hills: the Palatine, Aventine, Caelian, Esquiline, Viminal, Quirinal and Capitoline.

These hills are significant because they provided defensive positions for the early settlements and also influenced the city's development and expansion over time. The layout of the city's streets and neighborhoods reflects the topography of these hills and their valleys.