Archaeologists now have reason to suspect, and are digging to confirm, that the Roman fort at Lancaster was much bigger than previously thought, housing thousands of cavalry. Roman ships would have sailed up the River Lune and the fort would regularly have sent troops the short distance north to Hadrian's Wall, where we first meet Poul and Karen Anderson's Romano-British centurion, Gaius Valerius Gratillonius, who, in the course of their fictional narrative, becomes Grallon, the legendary last King of the city of Ys on the coast of Armorica/Brittany.
I was told this by Andrew, the tutor of our Latin class which meets in the historic Lancaster Friends' (Quakers') Meeting House. George Fox, the founder of Quakerism, was imprisoned in Lancaster Castle. Andrew, a member of the Choir of Lancaster Priory Church, was informed of the new dig at a Church Council meeting because the Church is right beside the Castle which was always known to be on the site of a castra (fort). This is what it is to live immersed in history. See here.