Constantinople is the old name for Istanbul, the great city located in what is now Turkey. The first urban settlement at the site of Istanbul was founded as Greek colony about 658 BC, led by a fellow named Byzas from Megara. The town was then called Byzantium.
In AD 330, Byzantium was reconfigured as the capital city of the Emperor Constantine the Great, head of the Byzantine Empire. It fell to the Ottoman Empire 1000 years later, in 1453. It was officially renamed Istanbul in 1923. The city has some of the most beautiful architecture in the world, from Greek, Byzantine, and Islamic civilizations.
For more history of Constantine and his city, see N.S. Gill's article on Constantine the Great.