MATERIAL CASTLE' Material Castle is a term used to describe a type of fortification made from SAND. It is a form of defense that has been used since ancient times, and is still used today in some parts of the world. SAND castles are built by piling up SAND and then shaping it into walls and towers. The SAND is then held together with water or other binding agents. This type of fortification is often used in beach areas, as the SAND provides a natural barrier against attackers.