"He had not as a child ridden with Rupert's horse or sat among the camp fires at Xanthus-side"

 Prince Rupert of the Rhine was King Charles's nephew and a famous Royalist cavalry commander in the English Civil War.

 

Xanthus is the river of Troy referred to in The Iliad (8:561) by Homer in the lines:

 

  midway between the ships and the river Xanthus

  A thousand camp fires gleamed upon the plain.