C_A, Badger and myself have slung from the ramparts of Danebury hill fort.
http://www.youtube.com/watch?v=xW912BD9gZMMy hypothesis is that the outer set of earthworks is not intended to be manned. It is there to force the attackers to bunch up and make a denser mass that it much easier to get hits with.
Your best attacking strategy is to gather outside the maximum range of the defenders then run towards the fort in open order so that there is plenty of room in between each attacker for slingstones to land harmlessly in. The large shield of that period would be raised over the head at about 45 degrees to provide protection.
Once the attack meets the outer earthwork they are slowed greatly and will bunch up. This is where the slingers at the inner earthwork can really get their kills since any slingstone into that mass will hit something, and if hitting the shield of one man, may easily be deflected into the ribs of the next man etc.
Once the attackers reach the stockade the defenders at the point of attack would switch to spears but defenders on either side would keep slinging, having the advantage of shooting from the side while the attackers shields are occupied from the front.