enclosed areas on the top deck of a ship placed across its breadth from side to side and extending any distance along the length of the ship. As distinct from superstructures, enclosed areas on the deck that do not extend from side to side are called deckhouses or cabins.
Depending on location, a distinction is made between bow, midship, and stern superstructures. The number and dimensions of ship superstructures characterize the architectural class of the vessel. (For example, a ship with three superstructures is a three-island ship, and a ship on which the bow and midship superstructures are connected is a ship with a long forecastle.) Ships without superstructures are called flat-deck ships. Superstructures with watertight enclosures increase the buoyancy of the ship, protect areas underneath from water penetration, and protect open decks from flooding in turbulent seas. Cabins for the crew and passengers are usually located in the superstructures, and on transport vessels superstructures are sometimes used for cargo.