For the sake of my plot, I thought, “Well, Kasia has to go with Xerxes to war, otherwise we’ll have too many years apart. I wonder if I can come up with a good reason for this?”
Then I did my research and laughed in glee. See, the Persians all brought concubines with them. Struck the Greeks as kind of odd, but there you have it. Voila! Problem solved. Kasia would have been one of many women traveling with the army.
I further found a single line that made me grin. Xerxes, apparently, rode frequently at the front of his army . . . except for those other frequent times when he would retire to his wagon. A few little words that wouldn’t strike anyone else as important, but since I had Kasia in that wagon . . . well. Perfect, eh?