How'd you get Palisade? (wooden stakes)

Hi folks,

So far, every battle I've ever fought, I've never seen Palisade on either side.
...Palisade... you know; a line of sharp wooden stakes in the ground, protecting your units.

Do you have to have certain unit types, or is it when an army attacks you, & and on certain terrain etc?

Thanks for reading.

A variety of longbowmen (even mercenery longbowmen IIRC), Lithuanian and Janissary archers can use them.
However, the cannot be deployed on paved roads.

Only certain units can have these. These are (England) Longbowmen, Yeomen Longbowmen, Retinue Longbowmen. (Poland) Lithauen archers (or similar). (Turkey) Janessary archers.

You have to deploy the stakes before the battle begins (while you're placing troops, it's their "special ability") and they can not deployed on hard ground