Dispelling Some Myths: Mediæval Murder Holes
Look up whilst you are visiting castles and you will often see voids in the overhead masonry associated with the defence of the building. These can take the form of defensive structures built onto the top of, and overhanging, the defensive walls known as machicolations, or a sort of miniature machicolation known as a brattice usually placed to defend a specific weak point such as a doorway (examples pictured right), or holes in the gate passage, known as murder holes. The popular story is that they were built so that the defenders could pour boiling oil down upon attackers.
Although it is not a myth that these holes were created to hurl items into the spaces below them, including projectiles, large stones and caustic lime, their uses were even more complicated. They could act as safe observation points from which the wall footings or a passageway could be seen. If fires were started, either accidentally or deliberately during a siege, the slots could be used to douse the flames with water.
Boiling oil was rarely used, however. It was prohibitively expensive, not often available in large enough quantities to be effective, would have been difficult to heat (it has a boiling point at 204°C), problematic to transport around the parapets, and could have been a fire risk in itself. Yet the use of boiling oil has a hold in the popular imagination even though there are only a very small number of scattered references to the use of hot oil, for example, at the siege of Orléans in AD 1428. There are far more accounts of boiling water, molten lead, and even heated sand (all of which could penetrate armour more easily than other weapons) being used.
For the most part, castles were rarely besieged, and murder holes were mostly left untested. In fact, many of them were intended to be nothing more than symbols of architectural prestige.