The House of Orange-Nassau held four towns that defined its identity: Breda in the Netherlands, Dillenburg in Germany, Orange in southern France, and Diest in what is now Belgium. Together these are the original sister-cities, and Diest is still twinned with all three, plus the Dutch towns of Buren and Steenbergen. The princely family's most famous member, William I of Orange - William the Silent, 1533 to 1584 - led the United Provinces' revolt against the Spanish Habsburgs and was eventually assassinated in Delft in 1584. His sons split the Orange legacy along religious and political lines. The line that retained the Protestant cause went on to become Stadtholders of the Dutch Republic and, after Napoleon's defeat in 1815, kings of the Netherlands. The line that retained Catholic faith faded back into the older Burgundian world. Both kept Diest. Both used it. Neither was finally allowed to keep it: in 1795 French revolutionary armies ended seigniorial rights across the region, and the town became simply Belgian.

The central square is one of the prettier Grote Markts in Brabant, surrounded by townhouses spanning the 16th through 18th centuries. The 18th-century city hall, designed by Willem Ignatius Kerricx, anchors one side; its basement houses the city museum, where visitors can see the armor of Philip of Orange and a portrait of his uncle Rene of Orange-Nassau alongside Anna of Lorraine. St. Sulpitius itself, built between 1417 and 1534 from the brown sandstone typical of the period, rises at the head of the square. Inside its turret hangs a famous carillon cast in 1671 by Pieter Hemony - one of the great bell-founder brothers whose work still defines the sound of low-country town centers from Amsterdam to Ghent. The Gothic Church of Our Lady, built 1253 to 1288 a few streets away, holds an impressive pulpit and high altar; the house called the Gulden Maan, in the street named for him, is where John Berchmans was born in 1599. He became a Jesuit seminarian, died at 22, and was canonized as a saint by Pope Leo XIII in 1888.

On the Allerheiligenberg hill overlooking the Demer river, the Citadel of Diest is the only brick citadel left in Flanders. Major Laurillard-Fallot designed it between 1837 and 1843 as a bastioned pentagon with sides of about 190 meters, surrounded by a dry moat with a drawbridge - pure 19th-century military functionality, embellished only by a neoclassical bluestone gate. After it was decommissioned as a core fortress in 1895 and as a railway-protection fortress in 1906, the place went through every conceivable secondary use: a disciplinary company through the early 20th century, an army depot in the 1930s, a German garrison during World War II, an Allied prison in 1944-45, then 63 emergency houses built inside its walls in 1946. From 1953 the First Parachute Battalion of the Belgian Army lived here; in 1968 two of its city-facing fronts were partially demolished for new construction. Since 1996 the surviving structure has been protected as a monument. The city has been searching for a use ever since.

Diest is still partly wrapped in its high earthen ramparts, which protected it through wars older than the citadel. The Schaffensepoort, Fort Leopold, the Zichem and Leuven gate-locks, and the citadel together preserve a near-complete picture of mid-19th-century Belgian military architecture - the network that the new Kingdom of Belgium built across its eastern frontier as soon as it was independent enough to fear invasion. The countryside around the city includes the towns of Deurne, Kaggevinne, Molenstede, Schaffen, and Webbekom, all merged with Diest proper in the 1977 reorganization that gave the modern municipality its 58.20 square kilometers and roughly 23,000 inhabitants. The Hageland region rolls out around it in soft hills and farmland; the Demer river still defines the city's southern edge.

Beyond Saint John Berchmans and the House of Orange, Diest has produced a quietly varied set of names. Peter van Diest, writing in the late 15th or early 16th century, is the probable author of Elckerlijc, the Dutch morality play that is the direct source for the English Everyman. The astronomer Luis Cruls (1848-1908) was born here before emigrating to Brazil, where he became director of the Imperial Observatory in Rio de Janeiro and led the commission that surveyed the future site of Brasilia decades before it was built. Liliane Saint-Pierre, the singer; Timmy Simons, the footballer with 94 caps for the Belgian national team; the rock band The Scabs; Olympic marathoner Marleen Renders; and the Paralympic athlete Marieke Vervoort, who medaled in London in 2012 and Rio in 2016 before her widely covered euthanasia in 2019 brought a public conversation about end-of-life choices in Belgium back into the international press. The list is short, the names are weighty, and the town keeps its inscriptions modest.