When the Castle was first built the first room you entered was the living room. Access to this was on the first floor through the opening between the great hall and the living room. This part is now open and connects the two rooms but back in the 13th century this access was located on the first floor so to get there was by a forestair.
A forestair is a wooden ladder or stairs which could be easily collapsible in case of attack. The door would have been primitive yet strong enough to protect the keep. You can still see the original hinges and post hole within the door portal which would have opened outwards onto the forestair.
When the door was closed it would have been attached to the inside with a chain and a large wooden bolt that was located within the wall and slid into place to secure the door so no one could get in.
This door would have been quite thick with a small hatch to see who was outside, and the inner side would have had thick metal hinge brackets attached to it along with fixings for the long wooden bolt to be slipped into it to secure the door in event of any enemy trying to gain entry.
The Forestair at Hunterston would have looked pretty much identiacl to this one. Made of wood and easily collapasible in event of attack.
As the original entrance is now part of the new extension there is no way to show what the forestair would have looked like in 13th century, however the image above shows what it would have looked like if located at side of castle.