Before there was a county there was a Roman town. Verulamium, on the gentle hill overlooking the river Ver near modern St Albans, was the third-largest city of Roman Britain. Sometime in the third century, perhaps around AD 250, a Romano-British soldier named Alban gave shelter to a Christian priest fleeing persecution, exchanged clothes with him, and was executed in his place on the hill that the city now calls Holywell. Alban became Britain's first recorded Christian martyr. A church grew on the execution site. Around it, eventually, grew an abbey, then a town, then a cathedral. The cathedral that stands there now is mostly Norman, raised at the end of the eleventh century from Roman bricks salvaged from Verulamium's ruins. You can still see the bricks in the cathedral walls. The Roman amphitheatre is a flat oval cut into a field on the edge of town, the empty seats long since carried away to build houses.

Hertfordshire was close enough to London for the medieval and Tudor courts to use it as a hunting ground, a refuge, and a stage. Hatfield House, built by Robert Cecil in 1611 on the foundations of the old royal palace where Elizabeth I had grown up, still stands behind its red-brick walls. Knebworth, with its gothic facade later added by Edward Bulwer-Lytton, hosts rock concerts (Robbie Williams played to 375,000 people across three nights in 2003). The Wars of the Roses turned the streets of St Albans into a battlefield twice, in 1455 and again in 1461. Berkhamsted Castle, the motte where William of Normandy received the surrender of the surviving English nobles before his march on London, is now an English Heritage ruin in a park. Stuart kings hunted at Theobalds in Cheshunt, and it was James I who in 1613 sponsored Sir Hugh Myddleton's New River project to pipe fresh Hertfordshire chalk-water 40 miles down to thirsty London. The New River still flows.

In 1898 a social reformer named Ebenezer Howard published a slim book called To-morrow: A Peaceful Path to Real Reform. He argued that overcrowded industrial cities and depopulated countryside were two halves of the same problem, and that the answer was a new kind of settlement that combined the best of both: low-density housing in green surroundings, with jobs and shops and culture close to hand, a protective belt of farmland around the edges. In 1903, a company of his supporters bought 3,818 acres around the parish of Letchworth in north Hertfordshire and started building. The first house was occupied in July 1904. Letchworth became the model for Welwyn Garden City (also founded by Howard, in 1920), for Hampstead Garden Suburb, for postwar British New Towns, for Canberra in Australia, for Tapiola in Finland, and for Mežaparks in Latvia. The county that gave the world the garden city idea also gave it the green belt that was supposed to protect such places.

When the Attlee government passed the New Towns Act in 1946 to relieve the bomb-damaged housing crisis of London, Stevenage was the first town designated under it. Locals were not pleased. Lewis Silkin, the minister responsible, arrived at Stevenage railway station to find the platform signs replaced with the word "Silkingrad." A referendum showed 52 per cent of the existing residents opposed the plan. Silkin pressed on. The pre-war village of Stevenage was bulldozed around its medieval core and a new town of six self-contained neighbourhoods was built around a pedestrianised town centre, the first in Britain. Children of Stevenage who grew up cycling its segregated paths went on to include the seven-time Formula One world champion Lewis Hamilton, the footballer Ashley Young, and the golfer Ian Poulter. The Old Town up the hill, with its high street of medieval coaching inns, still stands. The new town centre is now being torn down and rebuilt sixty years on.

From the 1920s onwards Hertfordshire became, quietly, one of the world's great film-making counties. The studios at Borehamwood, often called Elstree because of the village nearby, made the first three Star Wars films, the Indiana Jones films, and the original Kubrick run of films when Stanley Kubrick chose to live and work just down the road. EastEnders is still made there. Big Brother UK and Who Wants to Be a Millionaire? both filmed in the same complex. In 2001 Warner Bros. opened a new operation at Leavesden, in a converted Rolls-Royce aircraft engine plant near Watford, and made every one of the Harry Potter films there. The Making of Harry Potter studio tour now draws more than a million visitors a year. The 1995 Bond film GoldenEye was shot there too. The county that gave the world Verulamium and the garden city also gives it Hogwarts.

Modern Hertfordshire is a county of contrasts. Watford in the south-west, with its Premier-League-bouncing football club and its medieval shopping street under a vast modern intu mall, feels almost like an outer borough of London. Sixty miles to the northeast, Royston is a small market town under the Cambridgeshire border with a chalk-cut cave full of mysterious medieval carvings underneath its high street. Between them lie the cathedral city of St Albans, the old wool town of Hitchin, the chalk-hill country around Tring, the Roman barrows of the Six Hills at Stevenage, and the green wedge of the Lee Valley that stretches deep into east London. The county has more intact medieval and Tudor buildings than London does. It is also home to MBDA missile development, Airbus satellite construction, GlaxoSmithKline pharmaceuticals, and the headquarters of Tesco. The deer still appear, occasionally, in the woods.