We visited three castles on our trip. The first was Old Innerlochy Castle, in Fort William. This particular castle is unusual in that its configuration hasn't changed much since it was built in the 13th century. When we visited it was a stronghold for midges, the particular swarming black-fly-like insects of the highlands. Apparently, Queen Victoria didn't much care for this castle, but I rather liked it.
Urquhart Castle, on Loch Ness, was also visited by Queen Victoria, who found the ruin quite picturesque. This castle had a fractious history and is now largely in pieces. Some of the destruction was intentional: the gate-house was blown up by its last garrison of troops when they abandoned it during the Jacobite uprising so that the castle couldn't be used by the enemy; the rest is a combination of decay and the local people scavenging building materials.
The third castle on our itinerary was Inverness Castle, which, unlike the others, we didn't explore. It's a working court-house today, so, short of breaking the law, you can't really get inside.