Dun is a generic term for an ancient or medieval fort. It is mainly used in the British Isles to describe a kind of hill fort and also a kind of Atlantic roundhouse. The term comes from Irish dún or Scottish Gaelic dùn (meaning "fort"), and is cognate with Old Welsh din, from whence comes Welsh dinas (meaning "city").
In some areas duns were built on any suitable crag or hillock, particularly south of the Firth of Clyde and the Firth of Forth. There are many duns on the west coast of Ireland and they feature in Irish mythology. For example, the tale of the Táin Bó Flidhais features Dún Chiortáin and Dún Chaocháin.
Duns seem to have arrived with Celtic cultures in about the 7th century BC. Early duns had near vertical ramparts made of stone and timber. Vitrified forts are the remains of duns that have been set on fire and where stones have been partly melted. Use of duns continued in some parts into the Middle Ages.
Duns are similar to brochs, but are smaller and probably would not have been capable of supporting a very tall structure. Good examples of this kind of dun can be found in the Western Isles of Scotland, on artificial islands in small lakes.
Read more about Dun: Toponymy