Lucius Artorius Castus was a Roman officer who lived in the late second century. When his military caraer and exploits are compared with those of Arthur there is some similarity. The two known inscriptions that detail the career of Lucius Artorius Castus place him firmly in Britain as a soldier who at one point in his career held the title "dux". While Arthur's title dux was eventually supplanted by rex ("king"), dux was clearly his original rank, and the development from dux to rex follows a direct line of transmission.
Lucius Artorius Castus does not mirror every detail of the Arthurian legends, however the parallels between King Arthur and Lucius Artorius Castus are striking across a range of details, both large and small.
Lucius Artorius Castus lived at a time that was perhaps earlier with the time span of the Arthurian legends.