[@pugbutter] Well to explain this geographical anomaly, it's the same as explaining on who  build the many tunnels and vast areas underground. When the Bastij arrived and explored the desert, they found traces of an ancient long lost civilization. All that remained were ruins. In the middle was found a giant Pyramid, all but covered in sand which now serves as the underground part to the palace that stands above it.

The reason for the desert being formed on the ancient kingdom's grounds involve magic portals, sand demons an ancient war, all which left the land as it is today. The effect of the portal still lingers and as so it has preserved the climate through the ages. The portal is located deep underground below the pyramid. 

I could as well go for mountains but I liked the idea of my desert being magic of origin and without any mountains involved. 


As for the trade routes, initially I had planned for the Bastij to have formed a trading agreement with the main land, using the coast owned by the Bosandas, also in an agreement with them, but that thought flew in the air. Now I'm left with only trade routes within the continent. A fleeting thought came to mind that not everyone would be willing to have a history of their country trading with another one on the other side of the vast desert but meh, I place myself at the role-player's mercy on that note. So it's all up in the air at this point, I might have to return to the agreement with the Bosandas idea.[@Sisyphus]

Hell, maybe the previous civilization left behind might mountains of treasures underneath. Deus Ex-machina for my economic situation, I'm cheap like that.