SultanAhmet Meydani, in front of the obelisk.
"The Obelisk of Theodosius (Turkish: Dikilitas;) is the Egyptian obelisk of pharaoh Tutmoses III re-erected in the Hippodrome of Constantinople (known today as At Meydan&i; or Sultanahmet Meydan&i;, in the modern city of Istanbul, Turkey) by the Roman emperor Theodosius I."
Taken from wikipedia.
For more information go to :

http://en.wikipedia.org/wiki/Obelisk_of_Theodosius