A castle is, by strictest definition, a residence that is occupied by nobility and fortified against attack. The most common location for true, historical castles is in Europe, where over 900 examples still exist today. The term castle now can refer to a particularly extravagant mansion, or a building that has some of the architectural features of a stereotypical castle.
Common Architectural Features: Castles come in many forms, but there are certain features that can be seen that are most commonly associated with castles. Many of these have been recreated in modern architecture, even if the function of them is gone. This includes towers and turrets, which now are used to make a home look stately but were once used as lookouts and strongholds instead. Other common features include stone walls to prevent burning, and open inner courtyards to provide a sanctuary for villagers under attack.
Famous Castles Today: In the US, there are three famous castles that people come and see as tourist attractions. The first two are at Disney World and Disneyland. Both of these feature many of the classic architectural components of a German castle, but are not really designed with interiors worth touring. The third building is Hearst Castle, in Northern California, which is known for its grand furnishings, and is a castle due to luxury more than architecture. It still remains one of the largest private residences ever to exist in the United States, and is so large that you can take several different day-long tours and still not see the entire home.