You guys have it confused with light infantry stakes which can be placed in combat. The pointy fence is deployed by Line Infantry along with their trenchs in defense. Light Infantry get mines in defense.

Those fence aren't only good cavalry defense: If you're defending vs an army with no cannons, you can just set them up slightly outside your line infantry range so the enemy units have to jump over it to start shooting and we all know how bad the unit AI is about fences and the like.