So there is definitely a fee to actually enter the castle area, and you may feel like you can see the whole thing (since its a ruin) from the outside. And that's true - if you're on a budget, I'd be fine saying to skip the trip inside. But if you can swing it, what makes it worth the trip inside are two things: first, the view down the cliffs from inside the castle. What an awesome fortress it must have been. Second, and most importantly, is the mine. Yes, people were trapped in the castle and tried to dig their way out. Yes, the entrappers figured this out and tunneled their way in from outside to meet them and thwart the escape. And YES, these tunnels still exist and you can crawl in and see it all. And the little museum area is great for town and university history. The city was extremely important in the middle ages and to the Scottish Reformation, so it really is interesting material. But honestly what I love best is just sitting on a bench overlooking the castle sands, watching the surf beat away at the crag the castle sits on, and staring at the ruins, so if you skip the visit inside, at least make sure you take a while to really look at it. Goes well with a nice hot tea if the wind is blowing. And it WILL BE.
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