There are loads of castles from the well restored and occupied like Castle Duart and Mingary Castle, to the crumbling and very ruined Ardtornish Castle. Eighty are in Argyll alone, I understand, out of about 2000 in Scotland. For maximum castle density head for the Sound of Mull. Or take your pick from, in alphabetical order of anchorage:
Achadun Castle, Lismore: very ruined
Ardencaple Bay, Firth of Lorne: Ardfad Castle; hardly anything left
Ardtornish Bay, Sound of Mull: Ardtornish Castle, ruined, great viewpoint
Aros Castle, Sound of Mull: ruined
Canna: Coroghan Castle, very ruined but lovely beach
Castle Sween, Kintyre: massive ruin, plus caravans
Dallens Bay, Lower Loch Linnhe: Castle Stalker, picture post card, and does tours
Duart Castle, Sound of Mull: the epicentre of Clan Maclean, well worth a visit
Dunstaffnage, Lower Loch Linnhe: good example of medieval castle, quite a lot left
Gallanach Bay, Firth of Lorne: Duntrune Castle, privately owned, restored
Garvellachs, Firth of Lorne: Dun Chonnuill castle, hardly anything left, hard of access
Gylen Castle, Firth of Lorne: great position, a lot of ruin to inspect
Kinlochaline Castle, Loch Aline, Sound of Mull: privately owned, hidden away
Lagavulin, Islay: Dunyvaig castle, very ruined
Loch Breachacha, Coll: the old castle, restored, privately owned
Loch Buie, South and East Mull: Castle Moy, ruined but now stabilised
Loch Moidart, North of Ardnamurchan: Tioram Castle, disintegrating
Mingary Bay, Loch Sunart: Mingary Castle, now a boutique hotel
Oban, Firth of Lorne: Dunollie Castle, now less creepy than it was, a sanitised ruin of note
Port na Moralachd, Lismore: Castle Coeffin, ruined
Rum: Kinloch Castle, a must see before it is too late, but maybe it already is
Shuna, Lower Loch Linnhe: ruined
Shuna, Loch Melfort: ruined
South Shian, Loch Creran: Barcaldine Castle, wonderfully restored L-plan house, now an up-market B&B
Tarbert Castle, Head of Loch Tarbert, Kintyre: best view outside from afar
Torosay, Sound of Mull: Victorian Gothic, now closed to the public