"The barrel vault represents the most important part of the structure"
The Barrel Vault, but also known as the Dungeon is located on the Ground Floor of the castle and is shaped with a curved roof. This part of the building supports the main castle structure and the walls are roughly 5 feet thick.
The walls at this level are the thickest and combined with the barrel shaped roof makes the whole castle building extremely strong but also supports the upper levels without any problems.
When the castle was first built the vault would have been impossible to access by any means from the outside with all four sides completely shut off from the outside world, the only main access to the vault was through an opening in the living room directly above by means of a small hatchway no more than 3½ x 2½ foot in size. This access has a wooden door and bolted from the top only. A wooden ladder would have been lowered down into the vault otherwise its a 12 foot drop to the bottom.
The main purpose of the barrel vault would have been used to store foods and grain to keep them fresh for the residents of the castle and the floor covered in reeds and straw but it is also suggested that the vault was used as a dungeon and enemies were thrown down the hatch into the depths below until it decided what to do with them.
When the new extension of the castle was built in the 17th century a new doorway was cut into the thick wall and a sturdy locakable door was added. This would have made access much easier for the staff and storage of items to be accessed more readily.
In the early to mid 19th century an office building was added to the North facing side of the castle and in 1912 when it was knocked down another door was added through the wall creating a large double wooden door. By this time the castle was used as the main farming homested known as "Hunterston Mains" and the vault was used to store the farm machinery.
In 2001 this door was removed and the wall reinstated to what it would have once looked like when the castle was built.
Due to the shape of the roof and the thick walls the structural inegrity of the building was not affected in any way and thats the master of structural engineering of the 13th century