When browsing Wikipedia, I came across this illustration that displays some of well known siege weapons of Roman empire.
However, something in this illustration caught my eye and it was the second "Wooden shield/blocker"(sorry I don't know the technical term for it) that is on the ramp. I am referring to the wooden shield that's located right by the wall. What is the purpose of a wooden shield so close to the wall? Why didn't defenders simply set it on fire or destroy it with some other means if it was that close? Is the Illustration misleading about the actual position of the second wooden shield? Are there any references that suggest Romans(Or some other faction in history) did indeed placed wooden shields that close to the enemy fortifications?