While the western parts of the Roman Empire gradually succumbed to barbarian invaders, the Byzantine Empire survived Germanic, Persian, and Arab attacks (Map 7.4). Why didn’t one or a combination of these enemies capture Constantinople as the Ostrogoths had taken Rome? The answer lies in strong military leadership and even more in the city’s location and its excellent fortifications. Massive triple walls, built and well maintained by the emperors Constantine and Theodosius II (408–450), protected Constantinople from sea invasion. Within the walls huge cisterns provided water, and vast gardens and grazing areas supplied vegetables and meat, so the defending people could hold out far longer than the besieging army. Attacking Constantinople by land posed greater geographical and logistical problems than a seventh- or eighth-century government could solve. The site was not absolutely impregnable, but it was almost so. For centuries, the Byzantine Empire served as a bulwark for the West, protecting it against invasions from the East.