During their early conquests, the Arabs quickly overran most of the cities of Palestine, which then formed part of the Christian Byzantine empire. Many of these cities were dotted along the edge of the desert region, making them easily accessible to the Arab invaders. Lightly garrisoned, and with significant Arab populations who had long maintained a working relationship with their kindred from the desert interior, they, for the most part, fell without any significant resistance.
Caesarea was different. It was the Byzantine regional capital, with strong walls and a legion headquartered there. It was also a coastal port city, meaning it could be resupplied by sea. Arab chronicles narrating their early conquests describe a siege that lasted years - seven years during which the Muslim commanders became frustrated at their inability to make any progress.
When Caesarea finally fell, it was through treason - treason committed by a Jew. Here is an extract from the history "The Origins of the Islamic State" by Persian historian al-Baldhuri. Bear in mind that Muslim chroniclers often exaggerated the numbers of people involved in battles and that they were using the Islamic calendar. It is believed that Caesarea fell in 640 or early 641.
Muhammad ibn-Sa'd from al-Wakidi : When 'Umar ibn-al-Khattab made Mu'awiyah governor of Syria, the latter besieged Kaisariyah until he reduced it, the city having been under siege for seven years. Its conquest took place in Shauwal, year 19.
Muhammad ibn-Sa'd from 'Abdallah ibn-'Amir : Mu'awiyah besieged Kaisariyah until he lost all hope of reducing it. Previous to this, the city had been besieged by 'Amr ibn-al-'Asi and his son. When Mu'awiyah at last took it by storm, he found in it 700,000 [sic!] soldiers with fixed stipends, 30,000 Samaritans and 20,000 Jews. He found in the city 300 markets, all in good shape. It was guarded every night by 100,000 men stationed on its wall.
The city was reduced in the following way : A Jew named Yusuf came to the Moslems at night and pointed out to them a road through a tunnel the water in which would reach a man's waist; in consideration for which information, safety was guaranteed him and his relatives. Mu'awiyah sanctioned the conditions [made to Yusuf] and the Moslems entered the city by night, calling "Allah is great !"After capture by the Muslims, the Christians were murdered or taken into slavery.
The captives from Kaisdriyah. Muhammad ibn-Sa'd from al-Wakidi : The prisoners from Kaisariyah [Caesarea] amounted to 4,000. When Mu'awiyah sent them to 'Umar ibn-al-Khattab the latter gave orders that they be settled in al-Jurf. They were then distributed among the orphans of the Ansdr, and some were used as clerks and manual laborers for the Moslems. The daughters of abu-Umamah As'ad ibn-Zurarah, having been given by abu-Bakr two servants from the prisoners of 'Ain at-Tamr who were now dead, 'Umar assigned to the daughters two of the captives of Kaisariyah to take the place of the two dead servants.This history fits the now standard pattern in which Jews facilitate Islamic conquest by betraying the Christians they have lived among, a pattern later evidenced in Spain and indeed throughout the western world today.
It's noteworthy that modern historians tend to cover up the role Jews played in facilitating the Islamic conquest. Here, for example, Robert Hoyland, describes the capture of Caesarea.
The new commander of the Arab forces in Syria, Mu‘awiya ibn Abi Sufyan, recognized this challenge and so brought seventy-two siege engines, which hurled stones day and night. This went on from December 640 to May 641 until finally the massive encircling walls of the city were breached. Since the troops had obdurately refused to surrender, Mu‘awiya decided to make an example of them, and all the 7,000 stationed there were killed save those who had managed to escape by boat to Asia Minor.
Source: In God's Path: The Arab Conquests and the Creation of an Islamic Empire by Robert G.. Hoyland
Hoyland claims the Arabs captured the city by bombarding and breaking through its walls. Although he doesn't say so, this account comes from the 12th century chronicle written by "Michael the Syrian". It is extremely implausible. The Arabs were barbarous desert nomads. It is fantastical to think that they would have been able to construct or even effectively and efficiently operate siege engines well enough to breach the walls of a major fortified Roman city. The Muslim chronicle was written much closer to the events themselves by an author with much better access to the relevant sources.
This reworking of historical narratives to leave out the parts unfavourable to Jews is extremely common in the present day. Many of the early chronicle accounts of Islam's emergence describe close cooperation between Muslims and Jews and Jewish delight at Christian discomfiture. Insofar as modern mainstream historians discuss these episodes at all, they tend to dismiss them as untrustworthy due to the chronicler's supposed antisemitism. But the evidence of the chronicler's antisemitism is simply that he reported an incident that reflected badly on Jews. Thus we are left with a bizarre form of circular logic in which anyone who reports a fact unfavourable to Jews is immediately classed as an antisemite and their "antisemitism" is then invoked to dismiss whatever it is they have said. So criticism of Jews becomes immediately auto-invalidating. This is Unreason almost demonic in its power and intensity.