Coastal defence ships were warships built for the purpose of coastal defence, mostly during the period from 1860 to 1920. They were small, often cruiser-sized warships that sacrificed speed and range for armour and armament. They were usually attractive to nations that either could not afford full-sized battleships or could be satisfied by specially designed shallow-draft vessels capable of littoral operations close to their own shores. The Nordic countries and Thailand found them particularly appropriate for their island-dotted coastal waters. Some vessels had limited blue-water capabilities; others operated in rivers.

Armour or armor is a covering used to protect an object, individual, or vehicle from physical injury or damage, especially direct contact weapons or projectiles during combat, or from a potentially dangerous environment or activity. Personal armour is used to protect soldiers and war animals. Vehicle armour is used on warships, armoured fighting vehicles, and some combat aircraft, mostly ground attack aircraft.