The hippodrome was an immense construction 480 meters length and 117.5 meters wide; it could seat, according to estimations one hundred thousand spectators. It was build in 203 by the Emperor Septimus Severus and later on Constantine the Great extended and remodeled it.
As Haghia Sophia was the center of religious life, the hippodrome was the center of the civil activities. The interests and passions of the population was divided between theological controversy and chariots races on the hippodrome.
Today you can only see some monuments where the hippodrome was. The central line, or spina of the hippodrome was marked by obelisks and columns, three of which are still outstanding monuments at Sultan Ahmet square (see picture bellow: the red line is where the hippodrome was located, behind it you can see the Blue Mosque and Saint Sophia).
The first monument is the Egyptian obelisk (picture above). It was originally commissioned by the pharaoh Thutmose III (1549-1503 B.C.) who erected it at Deir el Bahri opposite Thebes in upper Egypt to commemorate one of his campaigns in Syria and his crossing of the Euphrates river. It was erected on its present site by Theodosius the Great in 390 A.C. and at the bottom of it there is a marble stone with some sculptured relieves regarding Theodosius.
The second monument is the Serpent column. The three bronze serpents were the base of a trophy that once stood at the temple of Apollo at Delphi. It was dedicated to Apollo as a token of gratitude by the 31 Greek cities which defeated in the Persian battle Plataea (479 B.C.). The column was probably brought from Delphi by Constantine the Great.
The third monument is a roughly build pillar of stone 32 meters high dated around the 4th century. It is know that the monument was formerly reverted with gilded bronze plaques. In 1204, during the latin invasion, these plaques were removed, melted down and minted.