No one knows exactly where all of the city gates to Byzantium were located, but the literature of the day speaks of many of them. In a feature for Hürriyet Daily News, Niki Gamm discusses Istanbul's walls and gates and their places in the architectural history of the city.
From the article:
The Theodosian Walls connect with other Byzantine walls that run down to the Golden Horn. Eğrikapı is found there in a wall built in the twelfth century by the Emperor Manuel Comnenes. East along the Golden Horn ran the city’s sea walls. It’s still possible to see some of them still standing and even an arched structure that stood as a gate. The sea walls were less well built than those of Theodosius because the Byzantines didn’t really believe that they could be attacked from that side. They felt that the chain they had placed across the entrance to the Golden Horn in the eighth century and the strong currents in the Sea of Marmara would preclude any enemy maritime attack from that side. There must have been nearly 20 gates in the sea walls with the last one in today’s Sirkeci district being the one where the great chain was kept.