[http://en.wikipedia.org/wiki/Cheval_de_frise]
Cheval de frise
The cheval de frise (plural: chevaux de frise) was a Medieval defensive obstacle consisting of a portable frame (sometimes just a simple log) covered with many long iron or wooden spikes or even actual spears. They were principally intended as an anti-cavalry obstacle but could also be moved quickly to help block a breach in another barrier. They remained in occasional use until they were replaced by wire obstacles just after the American Civil War.
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