During the Napoleonic Wars, skirmishers played a key role in battles; they attempted to disrupt the main enemy force by firing into their close-packed ranks and to prevent enemy skirmishers from doing the same to friendly troops. Because skirmishers generally fought in open order, they could take cover behind trees, houses, towers and similar items, thereby presenting unrewarding targets for small arms and artillery fire. Such tactics often made them vulnerable to cavalry. Some skirmishers had a minor sapper role by placing "cheval de frise" to deter cavalry.