The Castle Stronghold was a former base of the Pure Ones.
The stronghold was first built by humans, or Others, a species that went extinct long before the book series began. It lay abandoned for thousands of years until it was discovered by the Pure Ones, who adopted it as a base. However, owls from St. Aegolius Academy for Orphaned Owls discovered and raided it. Knowing that the owl world could not be informed of their existence, the Pure Ones abandoned it.
The castle had multiple stained glass windows, all but two of which were destroyed by the ages. Only the east wall still stood at the time the Pure Ones inhabited it, and only one tower. There was also a room containing a rotting throne with walls covered in mu metal, which especially suited the Pure Ones' tastes as they used a niche there as a "shrine" to contain three bags of flecks.