Floating fortresses are maritime structures designed to serve as defensive installations or mobile platforms. They may also be referred to as battleships, cruisers, destroyers, frigates, or warships. These synonyms highlight the various sizes and functions of these vessels, as battleships and cruisers are typically larger and more heavily armed, while destroyers and frigates are smaller and more maneuverable. Other terms include aircraft carriers, which serve as mobile airbases, and submarines, which are underwater fortresses. In addition, the term "flotel" may be used to describe floating hotels or accommodation vessels used in offshore industries such as oil and gas exploration.