Castle Keep

Okay, not a castle - not even the ruins of a castle. This is an iron age hill fort named Dunadd. This is the "courtyard" feature - outside the left of the frame and behind me you can see the remains of artificially build walls. To the right there's a pathway up to the top where the footstep of the kings can be found.
This hill fort is in ruins, it was abandoned in the early medieval period of Scottish history and is currently under the care of the wonderful folks at Historic Scotland.
Hill forts built in Scotland are very different from what you'll see in terms of a hill fort in England as further south "hill fort" usually refers to the Roman construction which is a neat square earth work, in Scotland the people made use of the natural rocky hills that provided natural protection which could easily be converted into fortifications.
Kilmartin Valley where Dunadd is located is a region very rich in Scottish prehistoric (and early historic) archaeology. You can read about Dunadd and the area (Argyll) on wikipedia to find out more.

Note on the category: I have placed this under "Other" since while it is classified in my mind as a "Monument" I don't believe the DA category means monument the same way my professors of archaeology do.