The Koepelgevangenis (Dome Prison) is a striking 19th-century circular prison in the Valkenberg neighbourhood, built in 1886 on the panopticon design — a central observation tower surrounded by radiating cell wings, so a single guard could theoretically watch every cell. It was used as a prison for over a century, most notoriously to house convicted Nazi war criminals after World War II.
The prison closed in 2014 and the building has been converted into a hotel and events venue, which means you can now sleep in a cell or eat in the prison yard. Guided tours of the historic interior are available. The domed roof and the radial architecture are fascinating even from the outside.