Explanation: Romans had a neat little trick that was trotted out from time to time where they would form a kind of testudo so their comrades could use the shields as a ramp. It was actually an effective tactic for assaulting low walls and camp ramparts! No need to clamber up while some sod stabs at your grasping fingers, you can hop up and face him like a MAN.
As you can see in the second picture, using military gear of one's comrades for a quite literal boost is still a tactic some 2000 years later!