Large cavalry shield or circular infantry shield. See Ian Barnes, The Historical Atlas of Knights and Castles (New York: Chartwell Books, 2007), p. 160.
Large shield used by archers and bowmen to protect them while reloading their weapons. They usually had a protruding ridge running along the shield . See 'Knights and Chivalry: Glossary', http://members.tripod.com/aaron_neilson/glossary.html