The impact of Constantinople’s successful defense at that time cannot be overstated. Not only did it save the Byzantine Empire from the same fate as Sassanid Persia, but spared a fractured and chaotic Europe from Muslim invasion for another eight centuries. One can only wonder of the consequences for Europe and Christendom had Muslims armies marched unchecked into Thrace in the late 7th or early 8th centuries. What is certain is that the Muslim tide, broken at it shortest approach, was channeled to Europe via another, much longer axis-North Africa. Crossing the Strait of Gibraltar, a Muslim army of 50,000 traversed Spain, crossed the Pyrenees and penetrated into the French heartland before finally being overcome by Charles Martel at Tours in 732. With its expansion stemmed, the Muslim world turned its energies to internal disputes that splintered the caliphate, providing medieval Europe a much-needed period of growth and consolidation. In the end, the same spirit of ingenuity that created Constantinople’s fortifications would prove their undoing. The weaknesses of the defenses must have been obvious, since a series of attackers, beginning with the Avars, had tried to exploit them. Interestingly, the salient problems lay along the strongest point-the Land Walls. At a point just south of the Blachernae quarter, a section called the Mesoteichion, the walls dip sharply into the Lycus Valley, exposing that area to enfilading fire from higher ground on the enemy side. Apparently, the trace of the walls owed itself more to the need to accommodate a growing population than a regard for the natural lines of terrain. Another problem, far more perplexing, was the region of the Blachernae Palace, a neglected salient in the original Land Walls. The fortifications there, while often improved, were never equal to those elsewhere in that area. Finally, the construction of the Sea Walls as a single-wall circuit reflected a reliance on natural obstacles and a navy. As long as the Byzantine fleet commanded the narrows of the Hellespont and the Bosphorus, an attack from that quarter was not to be feared. That situation changed dramatically, however, after 1071, the year that the Seljuks of Rum inflicted a decisive defeat upon the Greeks at Manzikert. As the empire passed into decline, the Byzantine emperors could no longer maintain an effective navy, and gradually had to rely on the protection of friendly maritime powers. As the Byzantine navy withered, Constantinople lay exposed to an assault from the sea.

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The challenge was not long in coming. The first Crusades were a marriage of convenience for a Christendom divided between the rival Eastern (Orthodox) and Western (Catholic) churches. During the Fourth Crusade that enmity erupted into open warfare when the Latins sought to exploit one of Byzantium’s many dynastic squabbles. While en route to Palestine, the leaders of the crusade, cash-strapped and never opposed to a little profiteering, took up an offer by Alexius, the son of the deposed and imprisoned Emperor Isaac II, to restore their throne. In exchange for overthrowing the usurper, Alexius promised 200,000 marks, generous trade concessions and troops for the coming campaign. The deal was struck and on July 17, 1203, the Crusaders attacked Constantinople by land and sea. That night, the usurper Alexius III, fled and the next day Isaac was crowned with his son as co-emperor Alexius IV. Their restoration would be short lived. In January 1204, resentful Byzantine nobles toppled the puppet rulers and brought Alexius III’s son-in-law, Alexius Ducas Mourtzouphlos, to the throne as Alexius V. With no hope of securing Byzantine cooperation for the campaign to the Holy Land from the defiant new emperor and seeing little chance of success without it, the Crusaders determined once more to take Constantinople. The Latins, with a decisive naval advantage thanks to the financial support and powerful fleet put at their disposal by Venice, decided to make a major effort at the Sea Walls. To provide an assault platform, they erected siege towers on their ships from which long spars were rigged as a kind of suspended bridge. As a ship approached the wall or tower to be attacked, the bridge was lowered and the knights would shimmy across. The task of leading such an assault must have been daunting. A knight, grasping for balance moving down a narrow platform high above a ship rolling at anchor, then lifting himself over the parapet, all while evading the arrows, cuts and thrusts of the defenders, was at the mercy of his circumstances. When their first attempt failed, the Latins launched a second assault with two ships tied together. That provided a more stable platform and the possibility of assaulting a tower at two points. A witness, Robert de Clari, described how the attackers gained a foothold: ‘The Venetian who entered first in the tower was on one of these suspended bridges with two knights, and from there, with the aid of his hands and feet, he was able to penetrate the level where the bridge provided access. There he was cut down; it was there that Andr d’Urboise penetrated in the same way when the ship, tossed by the current, touched the tower a second time.’

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