Commonly called pila muralia, this denomination seems incorrect to us. It would be better to call them sudis (sudes) which correspond better to Latin and their function. Sometimes the word vallis is used, and it is correct too.
Their main function is to serve as a field fortification for temporary camps, but they can also be used to make movable caltrop barriers.
When legionaries built their camp every night they spent in the field, they dug a trench outside the camp and erected a rampart (agger) with the earth taken from the trench itself. Once the rampart reached two-thirds of its final height, they planted their sudis. The rest of the soil was then added and packed around them to properly keep them in place and make them hard to remove for an attacker.
They are then tied together by a rope. The side of this mound turned towards the enemy is then covered with turf from the ditch.
From Caius Marius reforms in 104 av BC they are transported by legionaries with the rest of their package.