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YEDİKULE
"Seven Towers"
Coming into İstanbul from the airport, the first glimpse you'll get of its history comes as the taxi pulls up at the traffic lights just before the breach in the Theodosian City Walls that once provided it with such sturdy protection. This is the part of town called Yedikule after the huge but under-cared-for fortress that was built into the walls here.
The fortress is not visible from the coast road but you will be able to see, on the sea side of the road, the elegant Mermerkule (Marble Tower) that once stood right on the seashore before land was reclaimed to permit development. Its stonework is so different from that of the rest of the walls that it is possible it once formed part of a palace that has since been lost.
In 2013 Yedikule has been in the news because of controversial plans to grub up some of the allotments that have stood on either side of the city walls since time immemorial in favour of a landscaped park, a rather odd scheme given that there already is a park beside the walls where they start their path north from the sea.
Yedikule
A huge gate leads into the open ground at the centre of the fortress which was created by adding three new towers to the four that already existed shortly after the Conquest of Constantinople in 1453. The most interesting of these towers is the one called the Ambassadors Tower or Yazılı Kulesi (Inscriptions Tower). It was here that foreign offenders were imprisoned, some of them scratching their names into the walls as they whiled away their days of captivity. A French prisoner called Francois Pouqueville who was imprisoned here in the 18th century has even left a written account of his ordeal.
But the single most interesting structure in the entire fort is the ghost of something that no longer exists. Once upon a time there was a huge triumphal arch decorated with statues that crossed the Via Egnatia here; it formed a counterpart to the one that once stood at the easterly end where Divan Yolu now begins.
The arch was erected in 388 to celebrate one of the Emperor Theodosius' victories. Called the Porta Aurea (Golden Gate), it was apparently fitted with gold-plated gates that were only opened after a victory. In 408 the gate was incorporated into the City Walls. It continued in use right through until 1261 when Michael VIII Paleologos celebrated the expulsion of the upstart Latin rulers from town by riding through them. After that, the gates were sealed and never reopened. 
The bricked-up Golden Gate is visible in the Walls from Kazlıçeşme
Today you can still make out the shape of the arch set into the wall (it's even clearer from the outside) although there's nothing to indicate just how important it once was.
It seems unlikely that Yedikule will continue as such an understated monument for much longer given how much renovation is currently going on around town. It would make the perfect setting for a museum celebrating the history of the city walls.
Transport info
Buses from Eminönü run along the coast road and stop at Yedikule. It used to be more pleasant to get here on the suburban train service which had a stop at Yedikule but I'm not sure if it will survive the coming of the Marmaray.
Nearby areas
Zeytinburnu