Valcan

A desert and wasteland south of Troytonia.

Valcan is to the west of Austrelion, across a sea. It doesn't go north-south so much as east-west. In the east, closer to the border of hot, rugged Ommersia, it's desert, but in the west it becomes more like steppe. Western Valcan, being relatively fertile, allows Valcan to sustain massive armies for at least a short time. The desert there gives a natural border. To reach Western Valcan, an enemy army has only a few options, all difficult. It can march straight through the desert, at great cost and difficulty. It can march north of the desert, but this approach involves crossing the not-Himalayas. It can march south of the desert to Parradelphon, and take ships with the elves, but this is time-consuming, and many men cannot fit aboard the elvish ships. Besides, the elves are not to be trusted. An army could sail across the central sea to Khitay, and seek permission to march through the Heavenly Emperor's lands. But he does not control the khans of the interior, with their tribes of warrior horsemen.