A hoarding was a temporary wooden (shed-like) construction that was placed on the exterior of the ramparts of a castle during a siege.
The purpose of a hoarding was to allow the defenders to improve their field of fire along the length of a wall and, most particularly, directly downwards to the wall base.
The latter function was capably taken up by the invention of machicolations, which were an improvement on hoardings, not least because masonry does not need to be fire-proofed. Machicolations are also permanent and siege-ready.
It is suspected that in peacetime, hoardings were stored as prefabricated elements.
In some castles, construction of hoardings was facilitated by putlog holes that were left in the masonry of castle walls.
Some medieval hoardings have survived including examples at the North tower of Stokesay Castle, Shropshire, UK and the keep of Laval, France. The inner citadel at Carcassonne, also in France has reconstructed wooden hoardings.
Now you find it around construction sites. =]