A ruin you climb into, a ruin you walk up to, and a castle you tour.
"Castle" covers three completely different visits on this route — a free, unfenced ruin; a hilltop tower reached by a real walk; and a fully staffed, ticketed attraction. Knowing which is which before you arrive changes how you plan the stop.
Ardvreck Castle, Loch Assynt — stage 3
Built around 1490 by the MacLeods of Assynt, Ardvreck is now a broken but still-recognisable tower, standing on a small promontory into Loch Assynt with a roadside car park a short walk away. There's no fence, no ticket booth, and no set opening hours — it's simply there, which also means no one is checking that the loose stonework is safe to climb on; stick to the marked path and don't scramble on the highest remaining walls.
Its clearest moment in recorded history: in 1650, the Royalist general James Graham, Marquis of Montrose, was captured nearby after his final defeat at the Battle of Carbisdale and held at Ardvreck before being sent south to Edinburgh, where he was executed.
Castle Varrich, Kyle of Tongue — stage 4
Castle Varrich is a small ruined tower on a hilltop above Tongue village, thought to occupy a defensive site used by Clan Mackay for centuries before the current stonework. It's reached by a waymarked path from the village of around an hour round trip, with a steady but manageable climb — genuinely a walk, not a roadside stop, and worth allowing the time for rather than trying to fit it into a five-minute pause.
Dunrobin Castle, near Golspie — stage 5
Dunrobin is a different kind of stop entirely: the ancestral seat of the Earls and Dukes of Sutherland, still one of the largest houses in the Northern Highlands, with a fairytale turreted exterior that's mostly the product of an 1835–1850 remodelling rather than medieval original stonework. As a ticketed, staffed attraction it comes with formal gardens, falconry displays, and a genuine interior tour — the version of "castle" most visitors picture, and the only one on this list with an admission fee and set opening season.
Choosing which ones to prioritise
If you're tight on time, Ardvreck rewards the least effort for the most atmosphere — it's a two-minute detour off stage 3's road. Castle Varrich is worth it specifically if you want a short leg-stretch walk on stage 4 rather than another driving-only day. Dunrobin is the one to skip only if you're genuinely castle-fatigued by stage 5 — it's different enough from the other two that "I've already seen a castle" doesn't really apply.