Babylon: The Roman fortress which stood on the eastern bank of the Nile and guarded the head of the Delta. It was built by the emperor Trajan (end of the 1st century C.E.) midway between the old Egyptian cities of Memphis, on the eastern bank, and An or Heliopolis (the City of the Sun) to the west. Taken over by the Arab Muslims in 641 C.E., it became the nucleus for the later development of the Islamic capital of Egypt. It is unclear, however, whether any of the Coptic churches standing today near the remains of the fortress of Babylon existed before the Arab conquest. Many of them, such as St. Sergius, St. Bacchus, St. Barbara, incorporate early fragments within their 9-10th century structures.