This actually is plausible because-Random thoughts about the Relic Castle in hide tags.
In past games, the ruins have been located in places suited for human settlement. Ruins of Alph has ponds, and Solaceon Ruins are near a river (Adv Regi chambers, despite being called ruins iirc, are really just caves for the Regis, and the Sinjoh Ruins will be discounted for now.). But the Relic Castle? Smack dab in the middle of nowhere with no drinkable water in sight. Why would humans build a society in the middle of the desert with no water but the ocean nearby?
My theory is this: The area around the Relic Castle was once a habited region with many, if not all, of the necessities that humans need, and was connected with the Dragonspiral Tower to the north (with the presence of Golett there...). The people were advanced (seeing as they had Sygilyph and the Golett line to aid them) and worshipped a sun god (Volcarona). But in some Atlantis-type scenario, the people angered their god in some way and Volcarona came and destroyed the place (Heat Wave off of 135 Sp.Atk is damn strong.), leaving pretty much nothing behind.
the terrain in the desert is has many indentations that could have been streams or a river at one point.  I realize sand does move and tends to accumulate in dunes, but still there is solid ground at one level of the desert with greater depressions below.
		