The fortress with Seven Towers( Yedikule ) is located district of Fatih near the Marmara. During Byzantine period it was five tower, after conquest of Istanbul two tower was added by Fatih Sultan Mehmet.  It is now open to the public as a historical monument with a permant exhibition in the courtyart. The prison of Yedikule is as a museum.
The fortress, walls and prison with seven towers (Yedikule)- Google Maps

The Byzatntine period Theodosius built a triumphal portal at Yedikule, opening onto the road to the imperial palace.The Byzantine name Golden Gate, the gate where the emperors and armies entered the city triumphantly after returning from a victorious military campaign. It's name "Golden Gate" came from the gilding on its façade. During Byzantine period had five towers. Because of five towers, the byzantine period also said "penta prikyor". For Greek "Penta Prikyor" means five towers.The Ottomans added two towers by Fatih Sultan Mehmet. So It's name "Yedikule" came from seven towers. After it was later used as a storage, then part of treasury and later as a prison. It was used as a prison for high-ranking officials during the Ottoman Period. It's name Yedikule Zindanları. Now it is open to visitors as a museum.

You can go by IETT bus to Yedikule.
IETT buses
80 YEDİKULE - EMİNÖNÜ
80B YEDİKULE - BEYAZIT
80T YEDİKULE - TAKSİM

you can come from Topkapı to Abdi Bekçi stadium coast of sea. You can see walls of seven tower from coast of Zeytinburnu.

Seven towers ( Yedikule ) has seven towers :
1. South Pylon Tower (Güney Pylon Kulesi)
2. North Pylon Tower (Kuzey Pylon Kulesi)
3. Inscriptions Tower (Kitabeler Kulesi)
4. Ball Tower (Top Kulesi)
5. Ahmet III Tower ( III. Ahmet Kulesi)
6. Fortress Tower ( Hazine Kulesi)
7. Flag Tower ( Bayrak Kulesi)

Prison of Seven Towers ( Yedikule zindanları ) entry prices?
17.0.2012
Prices: 10 TL
Open Date
The museum is open from 9 am to 18:30 pm every day.