It is a common image that springs to mind when thinking of medieval warfare, and there is evidence that it was used to deter attackers. The Jewish defenders of Yodfat in AD 67 poured hot oil on the Roman besiegers, and heated oil is also mentioned as being used against the English at Orleans in 1428-29.
But oil was a valuable resource and it was probably too scarce to be anything other than an occasional weapon.
There were cheaper and more plentiful alternatives. Defenders would hurl down rocks, stones or even pieces of their own wall, or pour down boiling water or heated sand. Attackers might also be blinded with quicklime, a kind of medieval mustard gas that became caustic when it came in contact with anything wet.
They could be burned by Greek fire – a mixture of resin, pitch, sulphur and naptha that was notoriously hard to extinguish. In 1216, the French defenders of Beaucaire Castle lowered a sack of burning sulphur, which drove off the attackers with its noxious fumes.
However, the prize for ingenuity has to go to the defenders of Chester who, in AD 905, are said to have inflicted a stinging defeat on the Vikings by dropping the town’s beehives on them.