Indeed, the source for mystic powers in the air called Mana was very rich here. So rich that it was on par with places like above a spirit vein, a sacred temple or circle and the Navel of the Earth.
However, deciding that this was a different world only based on that would be jumping to conclusions and totally unreasonable. It was possible that they simply chose a place with a rich Mana density to activate the magic circle. Actually, that was the most plausible explanation.