Castles were fortified structures built during the Middle Ages predominantly for nobility and royalty. They had features like moats, curtain walls with arrow slits, and keeps for defense. The word "castle" derives from the Latin "castellum" meaning fortified place. Castles served as residences for royalty as well as military defenses. Famous castles include Windsor Castle in England, Cite de Carcassonne in France, Hohensalzburg Castle in Austria, and Malbork Castle in Poland, the largest castle by area.