Craigmillar Castle
What's amazing about this castle is that it's within the Edinburgh suburbs; yet, it's isolated enough that it feels like it's in the middle of the border country. Thanks to (what I assume is) a policy of not allowing building in the immediate vicinity of the castle, the buildings are surrounded by green fields and trees. The effect of this setting is to transport the visitor back in time to an era where the castle was a working fort. The views from the top of the buildings are spectacular. Also, the architecture of the building is quite interesting. Any visitor to Edinburgh should make the effort to drive the three miles from the city center to Craigmillar. Cost to enter is £2. There aren't any discounts for students.