When the Roman Empire fell, the Eastern Roman Empire survived and became known as the Byzantine Empire. It was able to withstand attacks due to its massive wealth, fortified capital of Constantinople, and use of Greek fire. However, the Byzantines eventually lost most of their territory to the Arabs and Seljuks. In the 11th century, the Byzantine emperor appealed to the Pope in Rome for military aid. In 1095, Pope Urban II launched the First Crusade to help the Byzantines retake the Holy Land from Muslim rule. Tens of thousands of Europeans took the cross and marched to the Holy Land, eventually capturing Jerusalem in 1099 after heavy fighting.