The Corner Walls pack is part of the Castle Builder System. It is made of "Dwarvenite" and includes some metal and magnetized parts. This item was initially available as an add-on to a pledge level during the Castle Builder System Kickstarter. The item comes in both painted and unpainted versions.
The Corner Walls pack comes with these 14 pieces:
(2) Corner Spacer
(4) Inside Corner Wall
(2) Corner Arrowslit Wall - Left
(2) Corner Arrowslit Wall - Right
(2) Corner Wall Platform
(2) Dungeon Corner Post