A portcullis is a gate made of metal or wood that can be lowered to protect the entrance of a castle or important building. It is usually raised and lowered using chains and attached to grooves in the castle walls. The bottom of a portcullis is often sharpened into points. Portcullises were used to defend castles in times of attack or siege, and are not often found today. A few working portcullises still exist in some historical buildings such as the Tower of London.