Istanbul's fearsome Fortress of the Seven Towers(Yedikule), 5.5 km (3.4 miles) southwest of Sultanahmet (map), was where the ambassadors of foreign powers that had displeased the Ottoman sultan awaited their fate (often in privation), and where at least one Ottoman prince awaited—and met—his death.
It was not always so. This great pile of stones was first built as the Golden Gate in the times of Byzantine Emperors Theodosius I and Theodosius II (408-450). The twin square towers of the Golden Gate are now just two of the seven towers of Yedikule.
After he conquered Constantinople in 1453, Mehmet the Conqueror added three towers to make it a fortress, which he used as a treasury and prison as well.
Perhaps the easiest, and certainly the cheapest, way to go to Yedikule is to take a suburban train (Banliyö Treni)from Sirkeci Station toward Halkalı (they run every few minutes). Get out at Yedikule, turn left out of the station, and walk 400 meters to the fortress.