By the time Theodosius II arrived on the throne in A.D. 408, the city of Constantinople was bursting at the seams. As Septimus Severus and Constantine had done before him, Theodosius set about fortifying and enlarging the city, and the alternately towering and crumbling red brick and limestone monoliths you see running north from Yedikule (you'll see it as you arrive into the city) all the way to Ayvansaray and the Golden Horn are what remain of Theodosius's Land Walls. Twenty-five years after their construction between A.D. 412 and A.D. 422, the walls all but collapsed in an earthquake, and with the Huns advancing from the east, the politically polarized factions of the city set aside their grievances and in a period of 2 months, not only fortified the wall, but built another exterior wall plus a moat. (Constantinople nevertheless wound up paying Atilla the Hun protection money for 10 years after his attack.) Extensions to the wall, which under Theodosius ended at Blachernae, were added later by subsequent emperors. This latter section, from Ayvansaray down to the Golden Horn, was the weak link in the chain, and not surprisingly, the point at which Mehmet the Conqueror's army breached the city.
All together, the system of land defenses represents an impressive period in military architecture. The original, Theodosian wall consisted of a main (inner) wall 5m (16 ft.) thick and 11 to 14m (36-46 ft.) high, punctuated by 96 towers from 18 to 20m (59-66 ft.) in height. The refortification added an outer, crenelated wall reinforced with 92 towers placed at intervals of 50 to 70 or so meters (164-230 ft.), set in alternating position with the towers of the inner wall. Between the inner and outer wall was an inner terrace embankment (called a peribolos). Another wider embankment separated the outer wall from the moat, creating a vulnerable open space for those (un)lucky enough to get that far. The moat was the first line of defense, presenting invading armies with a formidable obstacle 20m (66 ft.) wide and 10m (33 ft.) deep. All told (including the additional sections north of Blachernae), the total length of the land walls is 6,670m (4 miles). Entrance and exit into and out of the city was via monumental gates constructed of stone, marble, and even, in the case of the Golden Gate at Yedikule, of gold. One of the most important gates of both the Byzantine and Ottoman periods was Edirnekapi, or the gate leading to Edirne. Edirnekapi, which is part of the original Theodosian wall, provided access to the main thoroughfare into the city center. Byzantine emperors used this gate when leaving for and returning from their campaigns abroad, and Fatih Mehmet (aka "the Conqueror") made his victorious entrance here. Later Ottoman sultans, after the official inauguration ceremonies at Eyüp, followed in Mehmet's footsteps and also entered via this gate.