This rock in front of the coast already hosted a custom station at the times of Alcibiades the Athenian thanks to its ideal position at the entrance of the Bosphorus strait. A fortified tower on the same small island is now able to control all naval traffic coming from and heading to the Black Sea; in addition, a large chain can be lifted to the sea level between the tower and Constantinople, effectively blocking the passage to any dangerous or unwanted ship.

Source: game/common/buildings/00_special_buildings.txt, game/common/holdings/00_holdings.txt, game/common/script_values/00_building_values.txt and the English localisation, Crusader Kings III 1.20.0.3.