Trous de loup
In mediaeval fortification, a trou de loup (plural trous de loup) was a type of booby trap or defensive obstacle. Each trou de loup consisted of an inverted conical pit about 2 m (6 feet) deep and 1.2 to 2 m (4 to 6 ft) wide at the top. At the bottom of the pit, a sharpened wooden stake would be hammered in. In some cases, the pit was concealed by light cover of wicker and a layer of soil.
Even if the term has a medieval origin, this kind of device was first described by Julius Caesar, in the seventh book of his Commentarii de Bello Gallico (Commentaries on the Gallic Wars). Used during the siege of Alesia, Caesar calls them lilies for their resemblance to the flower of the same name.
Trous de loup might be found singly as a trap (in which case they were always concealed), or in a dense pattern with no gaps between pits, used as an obstacle in front of a defended position.
A field of trous de loup could be made most effective if subsequently flooded to a shallow depth, which would conceal the pits, make their sides slippery, and add the risk of drowning.
Etymology: Trou de loup is French for "wolf hole", and presumably referred originally to a trap set for wolves.
See also