When the empire of rome was destroyed, why did the eastern half of the empire (byzantine) survive? answers for dummies?

 It survived the fragmentation and fall of the Western Roman Empire in the 5th century AD and continued to exist for an additional thousand years until it fell to the Ottoman Turks in 1453. But why did it survive? The eastern half of the Roman Empire was far less vulnerable to external attack, due to its geographic location. With Constantinople located on a strait, it was extremely difficult to breach the capital’s defenses; also, the eastern empire had a much shorter common frontier with Europe. It also benefited greatly from a stronger administrative center and internal political stability, as well as great wealth. The eastern emperors were able to exert more control over the empire’s economic resources and more effectively muster sufficient manpower to combat invasion. 

The case was a u.s. supreme court decision that defined the scope of the u.s. congress's legislative power and how it relates to the powers of american state legislatures. the dispute in mcculloch involved the legality of the national bank and a tax that the state of maryland imposed on it.