Boiling oil is a favourite with Hollywood (and nowadays with computer war games, too). History provides some accounts of its use. The Jewish defenders of Yodfat (Jotapata in modern-day Lower Galilee) are said to have used it against Vespasian’s troops in AD 47. Other mentions include during the Hundred Years’ War siege of Orléans (1428–29), the Great Siege of Malta (1565) and the siege of Sommières in the French Wars of Religion (1573).
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Some authorities claim that oil would have been hot, rather than boiling, and that it would have had the added advantage of making the attackers’ footing more slippery. Oil was expensive, but the conventions of medieval warfare held that the inhabitants of a town resisting attack could be put to the sword – so defenders’ financial priorities would change dramatically! The real reason that oil was used rarely, one suspects, is that few places possessed enough of it.
Reports of boiling water and heated sand being poured on attackers are far more common in ancient and medieval warfare. (Hot sand getting into your armour is, by all accounts, a nasty experience.)
By the Middle Ages, machicolations and ‘murder-holes’ were essential elements of castle architecture, permitting defenders to drop things onto the heads of attackers. They were also essential for throwing water of whatever temperature on any fires the enemy may have started.
Answered by Eugene Byrne, author and journalist.