One of the finest artillery fortifications in Europe, Fort George was established in 1748 in the aftermath of the Battle of Culloden, as a base for George II's army of occupation in the Highlands. By the time of its completion in 1769 it had cost the equivalent of around £1 billion in today's money. It still functions as a military barracks; public areas have exhibitions on 18th-century soldiery, and the mile-plus walk around the ramparts offers fine views.
Given its size, you'll need at least two hours to do the place justice. The fort is off the A96 about 11 miles northeast of Inverness; there is no public transport.