I cannot verify any figures of casualties during the Middle Ages. No one counted the bodies; the figures are always impressionistic. The figure given reflects the writer's own views. '30,000 dead' at Jerusalem in 1099 means 'far too many'. But different writers give different figures.

To quote Dr John France, Victory in the East: A military history of the First Crusade (p. 355, on the massacre after the capture of Jerusalem):

'This notorious event should not be exaggerated. Many Jews survived; we hear of some being captured by Tancred and we know that some were later ransomed, while many Muslim refugees from the city later took refuge at Damascus bringing with them the celebrated Koran of Uthman. The shock expressed by Ibn al-Athir, for example, and his statement that 70,000 were killed, owes something to the later spirit of Jihad and the thirst for vengeance which it engendered. [Ibn al-Athir was writing a century later.] However horrible the massacre at Jerusalem, it was not far beyond what common practice of the day meeted out to any place which resisted. In 1057 the entire population of Melitene was slaughtered or enslaved by the Turks whose conquest of Asia Minor was particularly brutal, while in the chaos after Manzikert Greeks and Armenians slaughtered one another. Such events were not confined to the Orient: [William] the Conqueror's ravaging of the Vexin and sack of Mantes in 1087 was of such savagery that some saw his death in the ruins of this city as divine vengeance. In the 'harrying of the north' [of England] by the Normans, Ordericus [Vitalis] believed 100,000 Christians perished and, commenting on William's role in this 'brutal slaughter', remarked that 'I cannot commend him.'

In short, the crusaders treated the defeated city of Jerusalem exactly as they would have treated a defeated city in Europe. I am not sure how many people died when the British and Americans bombed Dresden during the Second World War, but I have heard a figure of 35,000 in one night; the motivation was similar.

It has been pointed out by one reader of this page that nearly a century later, in 1187, Sultan Saladin did not massacre the Christian population of Jerusalem when he captured the city on 2 October. Instead, he allowed the bulk of the population to ransom themselves and freed others for no payment, allowing them to leave for Antioch or Alexandria in order to embark for Europe. His action was praised by some contemporaries such as his ministers Bahâ' al-Dîn and 'Imâd al-Dîn, and also by the Christian writer of the chronicle attributed to Ernoul, although the Christian author of the first Itinerarium Peregrinorum criticised him for demanding too high a ransom and declared that 14,000 poor Christians were enslaved. The Muslim historian Ibn al-Athîr, however, criticised Saladin for allowing the Christians to escape alive, as they could then regroup their forces and launch a counterattack. As a result, Saladin lost control of the coastal cities and the Europeans retained a foothold in the Middle East for another century. His merciful action was humane but was bad strategy. Saladin's successors did not repeat his merciful actions: the great Mamluk sultan and general Baybars and his successors massacred or enslaved all the European Christians in the castles and cities which they captured. This was inhumane but it ensured that the European presence on the mainland was eradicated in 1291 and never returned.