These planks functioned as a chest protection for the warriors. Diagonal poles kept the whole thing in place. It has been calculated that for this rampart 12,000-14,000 oaks must have been used. The earth for the rampart came from the rampart diggings - 40,000 m3 in all. Nobody knows how many men worked on the ramparts.
Indications are that it was a rush job.
Probably 2000 men could have built it in a month.