Trou 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.
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.
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