Istanbul was an ancient Earth city located in Eurasia. In early history, it was known as Constantinople.
In the 13th century, one Crusade resulted in the Sack of Constantinople. (TOS: "The Squire of Gothos", DS9: "Q-Less", VOY: "The Killing Game")
In summer 1334, Constantinople was affected by the Bubonic plague while Flint was in the city. (TOS: "Requiem for Methuselah")
During the 19th and 20th centuries, the city was the destination of the Orient Express. (TNG: "Emergence")
In the Julian Bashir, Secret Agent holoprogram set in the 1960s, Istanbul sent several messages to an island in the South Pacific, that were intercepted by the Americans. (DS9: "Change of Heart")