The Byzantine Empire was created with the fall of Rome in the 5th Century, when the kingdom was divided into Eastern Roman Empire and Western Roman Europe. The capitol of the Byzantine Empire was Constantinople, or modern day Istanbul.
The city was founded by Byzantium. The Byzantine Empire is one of the most powerful and historically significant empires. Look at the modern map of Europe, and you’ll see countries that were united in one Empire.
Some of the achievements of the Byzantine empire include protecting Europe from eastern invasions, preserving Greek language, preserving Roman traditions, production of fine art with distinctive style, protecting the Christian Orthodox Church, their cities had plumbing which is still in use, and much more. Many of the churches in Greece and Turkey are either made during the Byzantine Empire, or inspired by it.
The empire lasted for more than 1100 years, from the 5th century, until the fall to the Ottoman Turks in 1453. During most of its existence, the Byzantine Empire was the most powerful economic, cultural, and military force in Europe.