A broch is an Iron Age drystone hollow-walled structure of a type found only in Scotland. Brochs belong to the classification "complex Atlantic Roundhouse" devised by Scottish archaeologists in the 1980s. Their origin is a matter of some controversy. The theory that they were defensive military structures (an Iron Age equivalent to the castles and tower houses of medieval Scotland) is not accepted by many modern archaeologists (see the 'general references' below), while the alternative notion that they were farmhouses is dismissed by some others. Although most stand alone in the landscape, some examples exist of brochs surrounded by clusters of smaller dwellings.