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.
You can easily reach Yedikule by suburban train, but you may want to consider an Istanbul city tour so you can also see other places in this area easily. (Details below.)
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.
The Golden Gate was the monumental and ceremonial entrance to the city as one came along the Roman road from Europe, with four massive towers built into Theodosius II's mighty land walls. Its gigantic doors were indeed once covered in gold.
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.
Yedikule is in the land walls near the Sea of Marmarashore. The land walls, by the way, extend for 6.5 km (4 miles) northward to Ayvansaray (near Eyüp) on the Golden Horn.
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.
You can also get here on good half-day Istanbul city tours operated by Backpackers Travel. The advantage of a tour is that you can see more in much less time. More...