The Long 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 Long Walls pack comes with these 4 pieces:
(2) Long Arrowslit Wall and Spacer
(2) Long Wall Platform