It consists of a large, medieval-themed stone castle with two balconies, a wooden drawbridge, a flowing moat, a main room with an entrance, a basement, and a large field with a clear, dirt pathway. The inner walls are stone bricks, while the floor is made entirely out of cobblestone.
- This was the first members-only den released.
- This den was used as part of the background for the April Fools' Party, but everything was turned upside-down including the castle itself.
- The Trading Party uses a background that is based on this den; the inside layout is the same, but the moats are slick with ice and the lawn is snowy with a different layout.
- This den is also used as the background for the medieval-themed Jammer Parties.
- Another castle-themed den is the Princess Castle, but its color palette and layout are different.
- With the King of the Jungle Membership Card (which is no longer available), the player had the option to choose either a free Castle Den or a National Geographic Kids Magazine Subscription.
- Unlike many other dens, there are no unique patterns for the wallpapers and floorings that can be used on this den.
- There is an unreleased room file of an incomplete version of this den.