Edinburgh Castle
In a trip full of wonderful things to see, this was probably our favourite. The castle is a wall enclosing ten or fifteen buildings, and they range in age from 900 years old to about forty years old. There's a huge amount to see. Catherine and I usually breeze through monuments in about an hour, an hour and a half, but we had nearly three hours here and it wasn't enough. It's a really fascinating place.
From the pictures towards the bottom you'll see that the Castle is built on a volcanic plug - practically unassailable in Medieval terms. It's still impressive.