In terms of geopolitics, perhaps the most seminal event of the Middle Ages was the successful Ottoman siege of Constantinople in 1453. Naturally, the Ottoman Empire would also use Constantinople as the capital of its empire after their conquest effectively ended the Byzantine Empire, and thanks to its strategic location, it has been a trading center for years and remains one today under the Turkish name of Istanbul. The end of the Byzantine Empire had a profound effect not only on the Middle East but Europe as well. Constantinople had played a crucial part in the Crusades, and the fall of the Byzantines meant that the Ottomans now shared a border with Europe. The Ottomans were viewed as a threat by the predominantly Christian continent to their west, and it took little time for different European nations to start clashing with them. In fact, the Ottomans would clash with Russians, Austrians, Venetians, Polish, and more before collapsing as a result of World War I, when they were part of the Central Powers.
Of course, it wasn't an accident that Constantinople survived for so long. Since ancient times, settlements have been protected by walls, ranging from simple earth fortifications, mounds, and ditches to sophisticated structures incorporating gates and watchtowers. The globe is replete with famous walls such as that built by Hadrian in the north of England and the Great Wall of China, which spans hundreds of miles across a vast landscape. Some of the earliest evidence of defensive walling can be found in Africa, and the Bible includes references to the walls of Jericho, dating back to at least the 8th century BCE. Walls have protected great civilizations and occasionally been their downfall, and the Byzantine Empire relied on Constantinople's extensive, sophisticated walls, among the greatest engineering achievements of the ancient world and Early Middle Ages, to survive as long as it did.
