Caesarea was an important town and port on the Mediterranean sea coast. Caesarea is now in ruins, but at the time of the invasion of Syria by the Muslims in the seventh century, it was a large city with three hundred busy streets. The city was populated by the Christians who were completly, hellenised.
As Caesarea was a large port and had communications through sea with Constantinople as well as Alexandria, it continued to receive help from the Byzantines. As the Muslims had no navy they were unable to intercept such aid. As help could come to Caesarea through the sea, and mustered there as a menace to the Muslims, it was even at the early stages of campaigning in Syria felt by the Muslim high command that Caesarea should be captured by the Muslims.
The city was first invaded in 635 A.D. under the command of Amr bin Al Aas. The siege dragged on for several months without producing any result to the advantage of the Muslims. On the eve of the battle of Yermuk, the siege was lifted to enable all the Muslim forces concentrate at Yermak.
After the conquest of Jerusalem, Umar issued orders that as Caesarea was the only Byzantine pocket left in Palestine it should be conquered. Yazid bin Abu Sufyan was appointed to the command. He marched with an army 17,000 strong and invested the city. A few days later he caught plague and withdrew to Damascus where he died.
After the virulence of the plague was over, Umar ordered that another campaign should be launched against Caesarea. This time Muawiyah bin Sufyan the brother of the former commander Yazid was appointed to the command. Muawiyah pressed the siege. There were occasional skirmishes but thanks to the help that could come to the besieged through the sea, the city held out.
The city was populated by the Christians but there was a sizable minority of the Jews in the city as well. Some disputes arose between the Christians and the Jews, and in order to seek vengeance from the Christians the Jews decided to help the Muslims in their fight against the Christians.
One day, a Jew Yusuf by name came to the Muslim commander Muawiyah and told him that if the Muslims granted amnesty to the Jews they would help the Muslims win the city. Muawiyah gave the Jews the amnesty they asked for. Yusuf accordingly told the Muslim commander of a subterranean passage that led to the citadel. At night a Muslim contingent crept into the city through the subterranean passage and overpowering the guards opened the gates of the city for the main Muslim army to enter.
The Christians fought to the last and tbey were killed in thousands. Brought to bay the Christians surrendered on the usual terms of paying the Jizya. Caesarea fell in 640 A.D., and that was the last Muslim campaign in Syria. With the fall of Caesarea the entire Syria came under the control of the Muslims.