Caer

The term is thought to have derived from the Brittonic *kagro- and to be cognate with cae ("field, enclosed piece of land").

[4] 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.

Note that the term is not believed to be related to the Irish cathair ("city"), which is instead derived from Proto-Celtic *katrixs, *catarax ("fortification").

[5][6] Gildas's account of the Saxon invasions of Britain claimed that there were 28 fortified Roman cities (Latin: civitas) on the island, without listing them.

[13] Some of the place names that have been proposed include: The element caer, sometimes anglicized as car, is found in several place-names in Wales such as: The Cumbric language was spoken in Northern England until the Medieval era in which the element caer ("fort") was used in naming places.

The north gate of Cardiff Castle , following the old Roman fortifications and rebuilt along Roman lines.
Roman Britain ( 1911 ).
Caernarfon derives its name from the Edwardian Caernarfon Castle
The Roman fort now known as Segontium derived its name from a latinization of the British community along the Afon Seiont [ 2 ]
Carriden House, a refurbished Roman fort which formerly formed part of the Antonine Wall in Scotland.