When armies stay motionless on the world map for a certain amount of time (one turn for Napoleon: Total War and two for Empire: Total War) and is attacked, certain units gain the ability to place useful objects on the field before a battle commences. Chevaux de Frise, consisting of a frame covered with many spikes, are available to a wide range of infantry, mainly militia and line infantry types. They are a barrier used to deter and kill enemy cavalry. Cavalry walking slowly into Chevaux de Frise suffer comparably few casualties, while cavalry galloping full-tilt into Chevaux de Frise can expect to suffer heavily. Chevaux de Frise are thus excellent means of area denial to enemy cavalry. They also serve to block musket and rifle fire, though they are not as effective at this as Earthworks and also block fire from one's own troops just as much. Friendly cavalry do not suffer casualties from Chevaux de Frise, though their mobility is hampered somewhat by them. Generally speaking, units that can deploy Chevaux de Frise also have the option to deploy earthworks (thus having to choose between the two), though a few units, such as Armenian Archers, can only deploy Chevaux de Frise. Certain units, particularly light infantry and skirmishers, also have the option of deploying Fougasse, a primitive land mine that is far more offensive in nature. The area denial against cavalry provided by Chevaux de Frise, though, may prove more valuable than the casualties inflicted by Fougasse depending on the circumstances.
Chevaux de Frise can be lengthened by stretching out the regiment deploying them before battle. This can help cover any holes in the defenses, and it comes with no downsides as the deploying troops are free to leave and reform after the deployment phase.
Chevaux de Frise are similar to Stakes in that both play an anti-cavalry role, but with some key differences. Stakes eventually break when they absorb too many cavalry impacts, while Chevaux de Frise are indestructible. Stakes are mostly harmless when approached from behind, but Chevaux de Frise are deadly on either side. Stakes are much less visible than Chevaux de Frise, making them much more likely to kill enemy cavalry controlled by an unsuspecting player. Finally, stakes are deployed during the battle phase, while Chevaux de Frise can only be deployed beforehand.
Chevaux de Frise are so named because the Frisian army, usually lacking cavalry, often made up for their cavalry deficiency by placing the eponymous obstacles on the battlefield.
Chevaux de Frise were used since medieval times, though similar devices may have been used as early as the Iron Age. They were in use during the periods Napoleon and Empire were set, and were still used in the Pacific theater during World War II, as a mount for barbed wire.