Several sets of walls were built around the city, beginning with the time of its founder Byzas, and they enclosed areas of differing size. Beyond the outer wall was a moat 10 m deep and 20 m wide, and inside this a second wall with 96 towers. As well as gates used by the general public, there were others reserved for military purposes. The walls overlooking the mouth of the Golden Horn where the city was least wlnerable to attack were the weakest. The next section to the south were the walls along the Marmara Sea which were 8260 m long and pierced by the Ahırkapı, Çatladıkkapı, Samatya and Narlıkapı gates. The land walls were 5632 m long and contained the Belgrad, Silivrikapı, Mevlevihane, Topkapı, Edirnekapı, Eğrikapı and Yedikule gates. Yedikule Gate was also known as Porta Aurea or the Golden Gate, and was the most magnificent, consisting of three archways. It was built by Emperor Theodosius (379-395). Over the gateway was a double headed Byzantine eagle carved in relief. It was through this gate that the emperors passed when returning from victorious campaigns. Istanbul’s city walls were almost invincible, and only breached twice in their entire history, once in 1204 by the Fourth Crusaders and once in 1453 by the Turks.
In 390 the Emperor Theodosius I had an obelisk brought from Egypt to Istanbul which he intended to erect as a mark of Roman supremacy. The obelisk dated from 1500 BCduring the reign of Pharaoh Tuthmosis II, and was one of two which stood at the entrance of the Luxor Temple in the city of Teb. The hieroglyphic inscriptions on the obelisk tell of sacrifices made to the god Amon-Ra. The obelisk was placed on the spina in the Hippodrome, on a rectangular marble plinth bearing relief carvings depicting Theodosius watching chariot races in the Hippodrome, and scenes showing how the obelisk was set in place.
Another monument on the spina of the Hippodrome was a bronze statue of three entwined serpents brought from the Temple of Apollo in Delphi. It had been made from the shields of Persian soldiers killed in the Battle of Palatea. Originally there was a gold cauldron resting on the heads of the three serpents, but this was apparently melted down for minting coins during the Latin occupation of the city, along with the bronze plates which covered the third of the ancient monuments on the spina, a stone pillar 32 m in height.