Open-air museums existed already in 1914, but they collected villages — farmhouses, barns, the rural past that nineteenth-century Europe was busy romanticising. Holm's museum did something no one had tried: it collected the market town, the købstad, with its merchants, workshops, customs offices and shopfronts. The distinction sounds academic and is not. Town life is where most Danes actually lived, and it is far harder to preserve, because towns rebuild themselves continuously. Nine years in, Holm heard from the National Museum that a large merchant's mansion in Aalborg — eight buildings in all — was to be demolished. He raised the money, moved the lot to Aarhus, and by 1926 five of them were open. At that point the project stopped being local, and the name changed to reflect it.

The town today is assembled from 75 historic structures drawn from twenty Danish townships, with three more in storage waiting their turn. The oldest is a storage house from Aalborg of around 1550; the youngest is a garden pavilion from that same 1909 exhibition. Between them stand a post office, a customs office, a school, a theatre, ten shops, thirty-four workshops and five gardens. The most ambitious rebuild took eleven years. The Coin Master's Mansion, Møntmestergården, went up in Copenhagen around 1683 as a residence for the royal coin masters and served that purpose until 1749. The Copenhagen City Museum took it down in 1944, handed the pieces to Aarhus in 1995, and reconstruction ran from 1998 to 2009 — rebuilt as the house is believed to have looked in 1750, furnished with period methods and materials.

Most heritage museums stop somewhere around 1900, as though history politely ended there. Den Gamle By did not. Alongside the 1864 neighbourhood there is now a 1974 neighbourhood, complete with Rahbek's mini-market, a shop called Pouls Radio and an embroidery shop, all furnished to the decade. There is a Museum 1927 — not a museum of 1927 but a reconstruction of what a museum looked like in 1927, glass cases and all, with genuine objects up to 10,000 years old inside them. Underneath the 1974 quarter sits Aarhus Story, an 800-square-metre exhibition covering twelve centuries of the city, told as far as possible in the words of people who lived here. The Danish name says it better: Aarhus Fortæller. Aarhus tells.

One three-room apartment in the museum is furnished as a 1950s home and is not open to the general public. It is the House of Memory, built for people living with dementia, along with their families and the staff who care for them. The premise is that the disease blocks the front door to memory but not the side entrances, and that sounds, smells and tastes can still get through. Visitors are encouraged to handle things, taste things and join in the housework — the opposite of everything a museum normally asks. The programme won the Luigi Micheletti Award in 2016 and, the same year, the award of the Nordic Centre of Heritage Learning and Creativity.

The collections inside the buildings reward slowing down. The Toy Museum holds some 6,000 pieces from the nineteenth century to the 1960s, including German Märklin cars and Bing trains, Danish Tekno models, and a quieter category: wooden toys made by inmates of Danish prisons. The silverware collection spans works by 310 smiths from 44 towns, from Niels Christensen around 1637 to Henning Koppel in 1918. The Pharmacy Garden of about 1750 is laid out as 91 baroque squares, each holding one medicinal plant of the period — henbane, thorn apple, belladonna, mistletoe, hop. The Renaissance garden of 1647 measures six metres by seven and was planted from a book, Hans Rasmussen Block's Horticultura Danica. Bergen and Turku later built museums along the same lines. Aarhus got there first, because one teacher refused to let a condemned house be rubble.