The name Acropolis means in Greek the Upper City. This is a reinforced hilled that was common in ancient Greece at the time. Actually, the Acropolis was originally used as residential areas for the residents of Athens. Its height and steep cliffs made the Acropolis into a perfect fortress, and protected the citizens against invaders. On the hills the kings would built their castles, and around them lived the commoners. Only when Athens' population grew, slowly people started moving to the foothills of the Acropolis, slowly making it the new religious center of the city. So at the Acropolis, more temples were built for the city.
This is exactly the area you are looking at now - a collection of temples and royal buildings from ancient Athens, where part of the buildings were built during the Roman period, like the library of of Emperor Hadrianus, that was built on the northern side of the Acropolis, in 132 BC
The meaning of the word Acropolis in Greek is - Above the city. See in Guidol's photos, how much the Acropolis of Athens really exemplifies these words.
Entrance is free on the first Sunday of each month. During tourist season (July-September), entrance is actually free on the second Sunday of each month. Additionally, between November 1st and March 31st, the entrance is free on every Sunday of the month.
Especially during the sunset, and 159 meters above sea level, the Acropolis is a great view point for the city of Athens, that will be laid out right in front of you.