This bewildering turret-topped building dominates the small German town of Bensberg, looking as if two completely unrelated structures crashed into each other on a misty hilltop.

With origins in the 12th century, the “Old Castle” of Bensberg was a hilltop fortress that saw several changes of use in its 900-year history, having already been a castle, a monastery, and a hospital by the mid-20th century. Then at the start of the 1960s, the civic dignitaries of Bensberg needed a new town hall (or Rathaus in German), and the idea was mooted to convert the then semi-ruinous yet clearly adaptable medieval castle into the new municipal headquarters.

The Old Castle had lain in the shadows of its palatial successor, the “New” Bensberg Castle, since 1706. But now it was time for the underdog of Bensberg’s pair of scenic citadels to take center stage with a bold, Brutalist reimagining.

The controversial conversion of the medieval castle into an aggressively angular town hall began in 1962, and was the work of expressionist German architect and concrete enthusiast Gottfried Böhm. The works began with the removal of post-1850 additions to the decrepit structure and the restoration of surviving 12th-century walls and towers.

The striking ‘60s additions to the site formed a horseshoe shape following the castle’s original footprint. The whole bizarre construction was curiously crowned by a cavalier concrete observation tower jutting high above its conjoined medieval ancestor.

The concept of a modern office complex, rising phoenix-like out of the ruins of a medieval castle was not without its detractors, even among the civil servants of Bensberg. They likened it variously to “bomb-damage,” “a garden gnome,” a “monkey-rock,” and a “public servants bunker.” The audacious building project earned its designer the coveted Pritzker Prize for Architecture and it remains a symbol of this small industrial town that still divides opinion.