This is an interesting question! It seems like not much has really been written about the Siege of Ascalon, either in primary sources or modern histories. Most of the time, it’s mentioned from the crusader perspective - the crusaders (or was they were also known, the “Franks”) successfully captured Ascalon, which allowed them to invade Egypt in the 1160s. But they were unsuccessful and the Fatimid caliphate ended up being conquered by Saladin, who then went on to destroy most of the crusader kingdom too.

From the Fatimid perspective, Ascalon was extremely important. It had also been the site of the final battle of the First Crusade, and although the crusaders were victorious there in 1099 as well, the Fatimids managed to hold on to the fortress. Egypt was able to launch invasions of the crusader kingdom from Ascalon for years after the First Crusade. The crusaders sometimes bypassed it and entered Egypt, but they couldn’t effectively invade Egypt as long as the Fatimids still had their outpost in Ascalon.

The importance of the 1153 siege really depends on a lot of other things happening around the same time. In 1144, the crusader County of Edessa, far to the north in Mesopotamia, was conquered by Zengi, the atabeg of Mosul and Aleppo. Zengi was then assassinated and his son Nur ad-Din succeeded him. A few years later in 1148, the Second Crusade arrived from Europe, but instead of recovering Edessa it ended up attacking Damascus, even though Damascus had been Jerusalem’s ally against Zengi and Nur ad-Din. The crusader attack failed.

Meanwhile, in Egypt, the Fatimid state was rather chaotic at this point and the caliph wasn’t much more than a weak figurehead. It had been this way since the time of the First Crusade fifty years earlier - the vizier al-Afdal had been in charge of defending Ascalon in 1099. In 1101 the infant al-Amir became caliph but al-Afdal continued to rule as vizier until he was assassinated in 1121. Al-Amir was assassinated too, in 1130. He was succeeded by al-Hafiz, who was an exception to the trend. His rule was relatively stable - well, at least, when he died in 1149, it was of old age and not murder!

He was succeeded by his 17-year-old son al-Zafir, who had no interest in governing, so once again Egypt was actually governed by the vizier, Ibn Masal. The death of the caliph and the succession of a new caliph always led to violence and rebellions by the army, and this time was no different. Ibn Masal was soon overthrown and executed by the governor of Alexandria, Ibn Sallar.

The Franks in Jerusalem knew all about this. After their failure in Damascus, they needed a big victory, and the western border was just the right place for it. They knew that Egypt wouldn’t be able to stop them from fortifying the border, so the king of Jerusalem, Baldwin III, fortified Gaza, just south of Ascalon. This caused some concern in Cairo, and Ibn Sallar suggested an alliance with Nur ad-Din, but then Jerusalem renewed its old alliance with Damascus, and Damascus prevented Nur ad-Din from doing anything to help in Ascalon.

In January 1153 Baldwin besieged Ascalon itself. Ibn Sallar sent an army, led by his stepson Abbas. But Abbas was more concerned with political intrigue back in Cairo. Abbas arraigned for his son, Nasr, to assassinate Ibn Sallar, and then Abbas was installed as vizier in April 1153. Abbas did end up sending some reinforcements to Ascalon but apparently he wasn’t too interested. It fell to Baldwin in August 1153.

Then the next year, in April 1154, Abbas and Nasr also assassinated al-Zafir as well as al-Zafir’s brothers and nephew, leaving al-Zafir’s infant son, al-Fa’iz, as caliph. And once again there were revolts and rebellions. This time, Abbas and Nasr were on the losing side. They tried to escape to Damascus, but were caught passing through the crusader kingdom. Abbas was killed and Nasr was sent back to Cairo, where he was beaten to death by al-Zafir’s surviving female relatives. Tala’i Ibn Ruzzik was appointed the new vizier.

Meanwhile, Nur ad-Din finally took Damascus in 1154, since the crusaders were now more preoccupied with the western border than their eastern frontiers. By now it was also obvious that Nur ad-Din was the real Muslim power in the area and Egypt was never going to be dominant or even an equal partner. Tala’i tried making an alliance with Nur ad-Din, but he wasn’t interested. The Fatimids needed his protection, but he didn’t need them.

“For the Fatimids, with the Franks in Ascalon, Nūr al-Dīn in Damascus and an infant in al-Qāhira, the events of the year from 1153 to 1154 were climacteric, the culmination of the threat to the dynasty’s position that began with the murder of al-Āmir and now left it fatally undermined and radically exposed.” (Brett, pg 283)

All of this culminated in the crusader invasion of Egypt in the 1160s, and a counter-invasion by Nur ad-Din, and the eventual destruction of the Fatimid caliphate in 1171 by Nur ad-Din’s general Saladin, who set himself up as Sultan of Egypt, inherited Nur ad-Din’s lands in Syria as well, and then conquered Jerusalem from the crusaders as well in 1187.

So, did the siege of Ascalon contribute to the fall of the caliphate? It was already weakened by numerous child caliphs and extreme instability, and it’s possible that Nur ad-Din would have intervened there anyway. But certainly, the fall came much more quickly because of the Frankish invasion, and the reason that the Franks could invade was because they took Ascalon first.

Sources:

Michael Brett, The Fatimid Empire (Edinburgh University Press, 2017)

Malcolm Barber, The Crusader States (Yale University Press, 2012)

Yaacov Lev, State and Society in Fatimid Egypt (Brill, 1991)

For primary sources, the two main ones are:

Usama ibn Munqidh, The Book of Contemplation: Islam and the Crusades, trans. Paul M. Cobb, (Penguin Classics, 2008) - Usama was present in Egypt at this time and claimed to be involved in all the scheming and murders

William of Tyre, A History of Deeds Done Beyond The Sea, trans. E. A. Babcock and A. C. Krey (Columbia University Press, 1943) - William wasn’t in Jerusalem at the time of the siege, but he has lots of information about it, and he was present for the invasion and the fall of the Fatimids