After leaving the lighthouse I started to make my way to the top of the island, an 1100 foot climb. A third of the way up I reached Ailsa Castle. For a short summary of what's known about this little stronghold see this link.
The ground and first floor were intact and I climbed the circular stairway as high as I could.
A window opening on the first floor gave a great view down to the lighthouse.
Time was short, and I still had a steep 700 foot climb ahead of me to get to the top of the island. So I left the castle and followed a trail that climbed straight up to another section of level ground. Here I came across the castle well. It was full of clear water, but with all the birds flying overhead I was not tempted to take a drink. My thirst, however, would soon be slaked by something much better. In my pack was a can of beer I planned to savour while sitting on the trig pillar at the summit.