Castle Byers was a makeshift fort made by Will Byers in the forest near his house. Filled with pillows and quilts and decorated with his drawings, it was Will's personal space where he could draw and read comics. Signs on the front read "Castle Byers", "Home of Will the wise" and "All friends welcome."
In the Upside Down, Will stayed in Castle Byers while hiding from the Demogorgon. Eleven used her abilities to locate and communicate with Will and told him Joyce was coming for him. A very weakened Will said she must hurry. Eleven told him to hold on a little longer, but she suddenly lost contact and Castle Byers disappeared along with Will.
As Will sang "Should I Stay or Should I Go" to comfort himself, the monster broke through the walls of Castle Byers, took Will and brought him to its lair in the library. When Joyce and Hopper entered the Upside Down, they found the remains of Castle Byers scattered across the ground.
- "Castle Byers" is also the title of a track on volume one of the Stranger Things soundtrack. The track was featured in "Chapter Seven: The Bathtub," as Eleven made contact with a dying Will in the Upside Down version of Castle Byers.
- The password to enter is "Radagast," the name of a wizard in J.R.R. Tolkien's The Lord of the Rings.