Since the perspective of the novel is through the eyes of the Persian King Xerxes, this historical novel shows the significance and power of the Greek army, especially the Spartan forces who played a role like the modern day elite forces. By bottle-necking the Persians into a small pass, the battle stole the advantage of numbers from the Persians. The Persians were forced to take turns fighting Greek forces that were far superior to them.
On a big battlefield, each Spartan soldier would have been separated by distance, allowing the Persians to surround each soldier and overwhelm them by sheer number and force, but in the narrow pass, the soldiers had to fight individually, without the ability to surround any soldiers, because to get around them would mean to defeat their lines, and no Spartan soldier was weak enough to allow such an advantage.
In terms of military history, the value of this from Xerxes's point of view is that he learned a painful lesson. Apparently, what he thought was going to be an easy victory was not, and he learned a lesson in strategy from the Greeks. To work with one's natural setting for the creation of advantages is just as important, it seems, as having the numbers to accomplish a goal. This is similar to the wisdom of Sun Tzu's "The Art of War," where he makes a similar argument, showing that natural advantages can change the outcome of a battle. History is full of examples like this.
