What was the "Fool's Tower" in Vienna until 1866?

The Narrenturm (Fool's Tower) in Vienna is continental Europe's oldest building for the accommodation of psychiatric patients. Built in 1784, it is next to the site of the old Vienna General Hospital, and is now home to the Federal Pathologic-Anatomical Museum Vienna.

The Narrenturm was used as a mental institution until 1866, at which point the tower could no longer house enough patients to cope with demand, and this, coupled with an increasing realisation that patients with such diverse needs should not perhaps be housed together, meant that the patients were then sent to newly constructed, purpose-built buildings, one of which was situated close by.