Scentillion is one of the five Monster cities in Verniculos Kingdom. It is located 36,000 miles away from the castle, which makes it 12,000 miles away from the south outer regions of the Demon cities surrounding the Demon King's Castle.
The Demon cities don't actually surround the Castle directly; they are surrounding Lost Cause Forest, the home of the thousands of weird ass Wild Beasts, which takes up the entire area between the Castle and the cities.
The forest acts as a defense measure. In case the Heroes have taken down every Demon city in Ilschevar Kingdom, which has happened for quite many times, the forest will slow them down so the Demon King will have an easy time to defeat them.
At least, that is the original function of the forest. Lost Cause Forest, however, has been proven to be useless 10 years ago when the Hero party easily passed through it like nothing—it only acts as the home of exotic Beasts ever since.