The Ahrensburger Tunneltal is not much to look at: a low, wet trough carved by meltwater running beneath a retreating ice sheet, now a stretch of grazing land and nature reserve at the town's edge. Between roughly 12,900 and 11,700 years before the present, during the cold snap known as the Younger Dryas, this was open tundra and the reindeer moved through it on their seasonal migrations. Hunters waited for them. The sites in the valley - Meiendorf, Stellmoor, Borneck - yielded the small tanged flint points that define the Ahrensburg culture, and at Stellmoor the remains of some 650 reindeer. Sunken carcasses, arrows still in place: this is not inference from scattered tools. It is a hunting ground preserved almost intact.

Alfred Rust (1900-1983) was a master electrician. He qualified in 1926, worked as technical director of an electrical firm until 1930, and then went digging in the tunnel valley, where in the 1930s he uncovered the reindeer hunters - not only the Ahrensburgian material but earlier Hamburg culture layers, evidence of people here some fifteen thousand years ago. It made his name and it made the town's. It also entangled him with the regime that was delighted to fund prehistory in the German soil: after his habilitation in 1942 Rust was appointed a corresponding member of the SS research organisation Ahnenerbe, and from 1943 he belonged to the Waffen-SS. Ahrensburg made him an honorary citizen in 1965. In 2000, planned celebrations of his hundredth birthday were called off once that record was aired publicly.

The modern town began under another name. Around the middle of the eleventh century a castle stood south of here - the Arx Arnsburga, also called Arnesvelde - and its ruins are still traceable in the Hagen forest; it appears on Ahrensburg's coat of arms. Reeves are recorded there in 1295 and 1304, but in 1326 Count John III of Schauenburg moved his reeve to Trittau and let the place go. The Schauenburg counts had meanwhile founded the village of Woldenhorn along with neighbouring Ahrensfelde, Meilsdorf and Beimoor. Woldenhorn is first named in the records in 1314; it passed to the Cistercians of Reinfeld Abbey in 1327 and served as the seat of the monastery's reeve until the middle of the sixteenth century.

Schloss Ahrensburg is the picture on every postcard: a compact white water castle with four slender octagonal towers under copper domes, raised around 1585 by Peter Rantzau, who had inherited the estate from his brother, the Danish general Daniel Rantzau. Between 1594 and 1596 Rantzau added the castle church, and beside it the Gottesbuden - two low almshouse ranges of eleven dwellings for elderly and infirm estate workers, an unusually humane provision for the period. By the mid-eighteenth century the Rantzaus were deep in debt. In 1759 the estate went for 180,000 Reichstaler to Heinrich Carl von Schimmelmann (1724-1782), who whitewashed the facades, filled in the moat, redid the interiors in rococo and laid out the street plan the town still follows. His money came from the Atlantic slave trade. From 1763 he owned four sugar and cotton plantations in the Danish West Indies, worked at first by four to five hundred enslaved people and later by around a thousand, and he ran fourteen ships in the triangular trade; he was Denmark's largest slave trader and among the richest men in Europe. The baroque symmetry of Ahrensburg was bought with their labour, and the town has increasingly said so out loud.

The railway between Hamburg and Lübeck arrived in 1865 and turned the village into an excursion destination; on 7 June 1867 Woldenhorn became an independent Prussian community and its council voted to rename itself Ahrensburg after the castle. The population reached 2,750 by 1910 and 17,775 by 1949, when Ahrensburg received town rights - about half of those people were refugees driven west from the former German territories in the east. In 1950 Erica Keck was elected mayor here, the first woman elected mayor in Germany. There are darker entries too. A small Jewish community lived in Ahrensburg into the early 1930s; the synagogue was burned during the pogrom of November 1938, and the Jewish cemetery, opened in 1822, still stands at the western edge of town. The castle itself has been a museum since 1935 and passed in 2003 to a foundation formed by the state, the district, the town and the local savings bank. It draws up to 30,000 visitors a year.