The Valley of the Kings was the heart of Odrysian Thrace (Thrake) in modern eastern Bulgaria near Kazanlak. The Odrysians were sometimes allies, sometimes enemies of Macedon, until Philip subdued them, then Alexander kept them subdued as a dependent kingdom. An Odrysian prince was a significant commander at the Battle of Issos. Earlier, while Alexander was still a prince himself, Philip led several campaigns into Thrace, including a three-year-long one. So there are several scenes in the novel that take place in Thrace, mostly in south Thrace, but the campaign against the Maidoi is further north. Like Lower Macedonia, Thrace is mountainous with wide valleys: horse country. Also, the burial customs of the Thracians and Macedonians display regional similarities with enormous wealth put into the ground for the royals, although Macedonian elite tombs contained the remains of the deceased while Thracian elite tombs--really cenotaphs--did not, following Orphaic traditions where the body was divided into parts and buried in various locations. A number of similar gods were shared across the two cultural groups, especially Dionysos. They may not have been ethnically/linguistically similar, but they certainly shared a number of cultural parallels.
In addition, I've put in one video made in Thessaloniki, of the modern statue of Alexander there by the harbor. Thessaloniki, named after ATG's little sister, was not built until after his death, so it extends well beyond the scope of the novels, but some readers may enjoy seeing it. And there are a couple of others, one about the roads in modern Greece (on the way to Dion), and another on the plane, saying "Bye."
Below the videos is the full-sized map of Macedonia made for the book by my niece, Selena Reames. All map copyrights are hers.
VALLEY OF THE KINGS, Bulgaria (Thrace)
MODERN THESSALONIKI (et al), Greece
MAP OF MACEDONIA, Selena Reames, artist