Dunstaffnage Castle
We assume this is the postern gate
A postern gate is another entrance to the castle -- often one that is down by a source of water, or near the shore that might give access to the castle for supplies or perhaps an escape hatch. They are usually well-guarded and well-secured, usually with an iron gate, sometimes with an offset passage.
just a small boat tied up on the shore near the castle
the plan of the first storey and the top story of the castle