Draw a line from Helensburgh in the south-west to Stonehaven in the north-east. South of it lies the Highlands. North of it, in theory, lies more Highlands. In practice, Aberdeenshire breaks the pattern. The terrain opens up onto a bedrock of Old Red Sandstone, low-lying and well-drained, where Aberdeen Angus cattle graze on grass that looks impossibly green against the bright red soil. To the west the Grampians do rise - cloud-wreathed, granite, ill-drained, poor of soil - but the county itself is mostly lowland. This split personality shows up in everything. The fishing ports of Buchan feel Nordic. The Dee valley feels Highland. Aberdeen feels like a city of merchants who happened to be issued mountains as well as harbours.

Aberdeenshire has more castles per square mile than almost anywhere in Britain, and the reason is wonderfully cynical. Any wealthy laird who wished to collect his farm rents in comfort while impressing the salons of London with his credentials as a Clan Chieftain required a castle in Aberdeenshire. The result is a remarkable density of them. Dunnottar perches on cliffs at Stonehaven. Drum and Crathes guard the Dee. Craigievar rises pink and turreted, the model for every storybook fortress. Balmoral, completed in 1856, was Queen Victoria's invention - all mock-turrets and stags' heads, the prototype of the Scottish Baronial style, and still the Royal Family's summer holiday home. Above Banchory, at Lumphanan, the real Macbeth made his last stand. Shakespeare took the same name and ran in a different direction with it.

Two industries reshaped the county. Herring came first - the "silver darling" that supported entire fishing communities at Fraserburgh, Peterhead and the cliff-clinging villages of Crovie and Pennan, the latter famous for the red phone box in the film Local Hero. Whaling sailed out of these same harbours in the eighteenth and nineteenth centuries, before the schools collapsed. Then in 1970, oil and gas were discovered far offshore in the North Sea. Aberdeen became the boom town that supported the rigs, with a stream of helicopters chattering overhead, ferrying workers out and back. Property prices became silly. Wealth spread unevenly across the shire. That industry is now in its sunset years, but it has left behind a culturally rich city and the helicopter routes that still trace the eastern sky.

Gaelic largely died out in Aberdeenshire by the nineteenth century, but Scots survived in a particular and gloriously dense form: Doric, the dialect of Buchan and the country corners. What makes it distinctive is the consistent substitution of "f" for "wh". "Fit like?" means "how are you?" "Far aboot ye fae?" means "where are you from?" Doric Aberdonians take a quiet satisfaction in being incomprehensible to Edinburgh and London alike. The first Bible translation into Doric appeared in 2012 - fifteen years after the Klingon edition. Today the dialect is more often deployed ironically in the city, as a badge of identity rather than daily speech. In the rural triangle of Buchan, it still flows as ordinary conversation.

Every town in Aberdeenshire seems to host its own Highland Gathering each summer - pipe bands, caber-tossing, the swirl of tartan against grey granite. The royal-attended Braemar Gathering anchors the calendar. The coast, meanwhile, offers a different rhythm: red sandstone cliffs at Gardenstown, the well-preserved old harbour at Portsoy, the ruins of Findlater Castle teetering above the Moray Firth. Whisky distilleries scatter through the county - Crathie, Macduff, Oldmeldrum, Huntly - though the true heartland lies west in the Spey valley. And then there are the small absurdities. Stonehaven's Carron Fish Bar claims to have invented the deep-fried Mars Bar, and still serves them. Aberdeen Angus beef is prized worldwide. The odds and ends, as they say locally, go into the burgers.