Isle of ....Mull, Iona , Cull, Tiree
...Skye a bit further north
Or walk in the highlands on the mainland.
Head northeast on the mainland to see Golspie, with a neat castle
(Of, you've already passed dozens of castles on your way, but this one is special)
Go way up northeast and see the Stacks of Duncansby
and yeah, make sure to bring a printed copy of the castle trail, there are too many castles to mention them all :p