thumb|right|The north gate of [[Cardiff Castle, following the old Roman fortifications and rebuilt along Roman lines.]]
Caer (; or ') is a placename element in Welsh meaning "stronghold", "fortress", or "citadel", roughly equivalent to an Old English suffix (-ceaster) now variously written as , , and .
In modern Welsh orthography, caer is usually written as a prefix, although it was formerly—particularly in Latin—written as a separate word. The Breton equivalent is kêr, which is present in many Breton placenames as the prefix Ker-.
Etymology
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 ' from its position on the Seiont; the later Edwardian castle and its community were distinguished as ' ("fort in Arfon", the latter being a district name (Cantref Arfon) from "ar Fôn", "(land) opposite Môn or Anglesey").
Britain
Gildas's account of the Saxon invasions of Britain claimed that there were 28 fortified Roman cities () on the island, without listing them. Silchester? "Here, says Nennius, Constantius the Emperor (the father probably of Constantine the Great) died; that is, near the town of Cair Segeint, or Custoient, in Carnarvonshire". Nennius stated that the emperor's inscribed tomb was still present in his day.)
- Cair Guricon (Warwick? thinking it to be the Roman Letocetum. Instead, excavations have shown that Letocetum was located at nearby Wall instead. Caersws? in Powys)
- Cair Mincip ("Fort Municipium": )
- Cair 'Pensa vel Coyt' ("Fort Penselwood":