One of the most important factors in a castle's defense was its walls. Very primitive castles and forts had timber palisades, but these eventually came to be replaced by the stone curtain wall. The curtain wall was extremely thick (about 10 feet) and may have been forty feet high. More often than not, the curtain wall had a spreading plinth -- an apron-like base which angled outward toward the ground. Not only did this add thickness and strength, it caused offensive materials dropped from above to hit the wall's base and splash outward at attacking soldiers. A parapet or allure (a walkway) ran along the top of the wall and was protected by a wall of alternating solids and voids. The solids, known as merlons provided archers and other defending soldiers a protective barrier. They also often had observation slits going down the middle. The then voids provided a gap through which to throw or fire materials. Sometimes the gaps or voids were replaced by shutters hanging on swivels in the merlons on either side. Shutters of this kind allowed archers to shoot down to the ground while protected from arrows or other materials falling from above.
Along the outside of the wall below the merlons were putlog holes. Timber beams slid into these holes and supported a wooden overhanging gallery known as hoarding. Hoarding was built all along the top of the walls and towers before a siege, allowing better command (more effective defense) over the castle's outer foundation. From inside the hoarding materials could be dropped or shot directly down upon the enemy.
Toward the end of the 13th century, permanent stone hoarding became increasingly common as wooden hoarding was easily set on fire by flaming projectiles (despite the watered down hides that often covered their roofs). These projecting masonry parapets were supported on stone brackets called corbels, thus replacing the wooden putlogs used in temporary hoarding. Between each corbel was open to the wall-walk forming a machicolation, through which offensive materials were dropped.
Often, stone hoarding extended not only over the outside perimeter of the castle, but over the inside perimeter. This way, if an attack penetrated into the bailey, the sealed walls still made an effective defense.