28 Cities of Britain

Caer (Welsh pronunciation: [kɑːɨr]; Old Welsh: cair or kair) is a placename element in Welsh meaning "stronghold", "fortress", or "citadel", roughly equivalent to the Old English suffix now variously written as -caster, -cester, and -chester. In modern Welsh orthography, caer is usually written as a prefix, although it was formerly—particularly in Latin—written as a separate word. The term is thought to have derived from the Brittonic *kagro- and to be cognate with cae ("field, enclosed piece of land"). Although stone castles were largely introduced to Wales by the invading Normans, "caer" was and remains used to describe the settlements around some of them as well. An example is the Roman fort at Caernarfon, formerly known in Welsh as Caer Seiont from its position on the Seiont; the later Edwardian castle and its community were distinguished as Caer yn ar Fon ("fort in the land opposite Anglesey"). However, the modern names of the Roman fort and Edwardian castle themselves are now Segontiwm or Castell Caernarfon, while the communities carry on the name caer.

...
Wikipedia