When talking about an empire, its location kind of becomes redundant after a while because you’re pretty much everywhere and you have your hands in all the cookie jars. Nevertheless, knowing where to put your capital city is essential no matter how big you are.
For the Byzantines it all started in 330 AD, when the Roman Emperor Constantine I moved the state’s capital from Rome to the newly founded city of, you guessed it, New Rome, later to be named Constantinople and what is now present-day Istanbul. Its location was excellent! It’s right on top of the strait of Bosphorus, which connects the Black Sea to the Mediterranean and separates Europe from Asia. From here you can literally keep an eye on what goes in and out of the empire, what others transport to and from their kingdoms through your backyard, and how much you can tax them for doing so.
Besides its economic advantages, having the capital in the east was a major plus because the east was where all the good fighting was going on. With the Bulgarians to the north and the Persians and later all the Islamic Caliphates to the east, keeping these wealthier provinces safe was essential and by no means easy. Having the capital so close to danger would seem counter-intuitive, but back then sending messages back and forth between the capital and the frontline would take weeks or even months and shortening that time was strategically crucial. Information is power!
Defending your capital from would-be invaders is a great example of forward thinking. And what better way to do so than by building the largest wall Europe had ever seen? The west end of Constantinople had no natural protection from invasion, so Emperor Constantine the Great started building one in 324 AD. Not even a hundred years later, Constantinople outgrew its boundaries and Emperor Theodosius II started construction on a second wall one mile west of the old one, which spans from the Sea of Marmara to the Golden Horn.
But disaster struck on November 6, 447, when a powerful earthquake destroyed large parts of the wall and none other than Attila the Hun was looking for some easy pickings. Luckily, the urban prefect Kyros of Floras managed to not only rebuild but also add a second outer wall and a moat, and all within 60 days. It was just in time to properly greet poor old Attila, who had to go back to pillaging the rest of Europe. These walls stand to this day as a testament to Byzantine engineering and are known as the Theodosian Walls.
Constantinople also had sea walls that completely surrounded the city. These weren’t as big or as fortified as the main walls, but access to the Golden Horn was restricted by a heavy chain and strong currents on the Marmara coast made an effective attack by a fleet next to impossible.
Constantinople also suffered from a lack of fresh water. The Valens Aqueduct, which was built by Emperor Valens in the late 4th century and still stands today, was 1061 yards long and the main source of water for the capital. The entire system of aqueducts and canals spanned a total distance of over 155 miles, making it the longest ever built in Antiquity. Together with over one hundred underground cisterns, which could house over one million cubic meters of water, Constantinople was an almost impenetrable bastion that could hold out against a siege indefinitely.