The Battle of Ascalon was fought on 12th August 1099. It was the last battle of the first crusade, but accounts of it vary. The crusaders had already taken Jerusalem when they learnt that a much larger force, led by Vizier Al-Afdal-Shahanshah, was approaching. Godfrey of Bouillon led the crusader army out of Jerusalem to meet to meet the Fatimid army.
The Wikipedia account of the battle describes the crusader army taking the Fatimid army by surprise while they slept. The Fatimids had not posted sufficient sentries to warn of the attack in time. This is how an army of around 10,000 was able to slaughter an army of around 20,000.
An account of the same battle on thoughtco, suggests that the crusader army came across a large herd of livestock en route to Ascalon. They brought the livestock with them and the dust cloud raised by the livestock led the Fatimid army to believe that the crusader army had been heavily reinforced. The crusader army were far better trained than the Fatimid army and perhaps aided by the sense the Fatimids had that they were outnumbered, the larger force were defeated.
Whenever I want more detailed research, I turn to JSTOR. A paper from John Hopkins University Press written by Dana Carleton Munro, includes a contemporary account of how a large herd of camels, oxen and sheep, accompanied them into battle and the dust cloud thrown up by them helped protect the crusader army.
Accounts agree that despite the crusader army defeating the Fatimid army, the crusaders failed to capture the city of Ascalon because of a quarrel between Godfrey and Raymond, Count of Toulouse.
Sometime in the early eighties, not long after I graduated, I was working at the Esso Fawley oil refinery. I went with colleagues to my first pub quiz and the first question was: who led the first crusade? I had only just started reading a book on the first crusade. I’m not even sure if I had finished the first page, but I was able to call out Peter the Hermit. We should remember that Peter participated in the massacre of between 2000 and 10000 Jewish civilians as they passed through France and Germany on their way to the Holy Land. Anti-semitism is the theme of Called to Account, the fourth book in the Sir Anthony Standen Adventures.
Incidentally, the photograph is from Jordan rather than Jerusalem, or Ascalon. It looks as though it is of the right period though.