Trenches and chevaux de frise are available right from the start, as are the artillery defenses. Later on light infantry get fougasse, essentially minefields. Light infantry can also plant stakes, which I believe are essentially chevaux de frise that can be deployed once the battle has started.

As for when you can use them, I believe it has to do with being attacked while your army still has movement points remaining (apart from light infantry stakes, which are always available). Make sure to move your general carefully when there are anti-cavalry obstacles around, I've forgotten about them and marched him over them before; he won't make any effort to avoid them.