Hidden doors are secret entrances that are often concealed within walls, furniture, or other structures. They can lead to hidden rooms, passageways, or storage areas, and are commonly found in historical buildings, castles, and even modern homes. These doors are designed to blend in with their surroundings, making them difficult to detect. The concept of hidden doors has been popularized in literature and film, often associated with mystery and adventure. They can be used for various purposes, such as creating a private space, enhancing security, or adding an element of surprise. Examples include bookcases that swing open or trapdoors leading to underground areas.