The Wilderness Castle can be found near the road to Sargoza in the Wilderness of Betancuria. It is surrounded by a high wall with turrets and has a two story keep and towerhouse in it's yard. The original purpose of the Castle was presumably to control the main road, but it was abandoned following the Dhorn invasion of the city and the subsequent collapse of the bridge over the Great River. The castle is now inhabited by a small group of bandits. The Princess passes near the Castle during Lesson 7.
Note: In Part One v1.30 and earlier, the bandits were hostile and the Princess could stealthily explore the castle at will. v1.40 and later requires interaction with the bandits before she can enter.
Spoiler: The castle is inhabited by bandits and in it's dungeon can be found a ranger, called Anden, who has been captured by the bandits. Anden can help the Princess to find a way to get to Betancuria and plays latter an important part in the story.