Panathenaic Stadium is located in Athens city center. Now it open again for the public. With an entrnace fee of 3.00euro you will have the opportunity to visit the stadium , which hosted the 1st Olympic Games of the modern era, in 1896. However, its history goes way before the 19th century AD...  A comentary auto-guided tour in 10 languages is available. 

The site of the Panathenaic Stadium was originally a small natural valley, between the two hills of Agra and Ardettos, over Ilissos river. It was transformed into a stadium by Lykourgos in 330-329 BC for the athletic competitions of "Panathinea", the greatest festivities in ancient Athens(the major Athenian festival in honor of the patron goddess Athena). 

 

Between 140 and 144 AD, Herodes Atticus restored the Stadium, giving it the form that was found at the 1870 excavation: the horseshoe construction with a track 204.07 meters long and 33.35 meters wide. It is believed that the Stadium had a seating capacity of 50,000 people.

In the Roman times it was used as an arena with the addition of a north semicircular wall.
The modern times restoration of the Stadium was conducted by G.Averof at the end of the 19th century, for the first Olympic Games that were reborn in 1896. The Stadium was rebuilt with marble from Mt Penteli, the same kind that was used 2,400 years before, for the construction of the Parthenon on the Acropolis. It could now hold over 60,000 spectators. The total cost was 1 million GrD, a huge amount of money in those days.

The Panathenaic Stadium holds a unique world record: on April 4 1968, AEK Athens won the European Cupwinners' Cup (beating Slavia Prague) in front of 60,000 spectators, the largest crowd ever to attend a basketball match.