Istanbul is the largest city in Turkey, constituting the country's economic, cultural, and historical center. The city is situated at the Bosphorus – one of the world's busiest waterways – in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its commercial and historical center lies in Europe, while a third of its population lives on the Asian side. The iconic landmark of Istanbul is the church of Aghia Sophia. This supreme example of Byzantine church building was constructed in the 6th century AD, in the reign of the Emperor Justinian. Its 11th Century wall paintings and sculptural decorations are considered masterpieces of Byzantine art. The Rumeli Castle is an imposing fortification at the narrowest point of the Bosporus, the castle was constructed a year before the fall of the city by Mehmed the Conqueror, to prevent any reinforcements reaching Constantinople from the East. The Archaeological Museum is one of the most important museums of its kind, and home to more than a million exhibits, from ancient Greek times through the Byzantine era to the Ottoman Empire. Not to miss are the Walls of Theodosius. These are the first great historical structures that greet the visitor to Istanbul, as he enters the Byzantine part of the city from the East. From the Sea of Marmara to the Golden Horn, the Walls of Theodosius have protected the city since 448 AD and constitute its most important defensive feature. Istanbul Ataturk Airport is used for private flights and is located about 40 kilometers from the city center. Sabiha Gocken can also be used and is located around 45 kilometers from the city center.