Dun (from the Brythonic Din (moern Welsh Dinas) and Gaelic Dùn, meaning fort) is now used both as a generic term for a fort (mainly used to describe a sub-group of hill forts) and also for a specific variety of Atlantic roundhouse. In some areas they seem to have been built on any suitable crag or hillock, particularly south of the Firth of Clyde and the Firth of Forth down across the border into Northumberland.