The Eterna Mountains
The range where the gate appeared, ceded as demon territory on the human side under the Treaty of Hope.
History
The range lay within the borders of the Kingdom of Lindsey before the war. Its transfer to demon control humiliated Lindsey's king and set the First Transgression in motion.
The Tower stands on its highest peak, visible from the windows of the royal apartments in Hikari and from a balcony of the castle at Kenmeikyo.