Segontium (Old Welsh: Cair Seiont) is a Roman fort on the outskirts of Caernarfon in Gwynedd, North Wales was founded by Agricola in 77/ 78 CE after he conquered the Celtic tribe of the Ordovices. The fort survived until the end of the Roman occupation of Britain c. 394 CE. It was garrisoned by Roman auxiliaries from Belgium and Germany.
According to Nennius “Here, Constantius the Emperor died.” Constantius Chlorus, the son of Constantine the Great, however, actually died at York. The Welsh monument could, however, be referring to Constantine, son of Saint Elen, a Celtic saint.
The original timber structure was replaced by stone in the first half of the 2nd century CE. Following the Norman conquest in the 11th century, they built a motte-and-bailey castle nearby and this settlement formed the centre of ancient Caernarfon.
After further Edwardian conquest in the 13th-century, this was replaced by Caernarfon Castle. Unlike the former fort, which was constructed on higher ground, the castle became a sea fortress and still looks impressive to this day. Some of the stones were actually removed from the fort to construct the castle. However, the remains can be seen to this day.
Segontium is referenced in the prose of the Mabinogion In “The dream of Macsen Wledig”, Macsen (Emperor Magnus Maximus) dreams of a beautiful woman (Saint Elen) who turns out to be at “the fort at the mouth of the Seiont”.
If you ever plan a visit to North Wales be sure to check out Segontium Fort and Caernarfon Castle.