While downstairs houses a small and rather idiosyncratic museum, it's Ibsen's former apartment, which you'll need to join a tour to see, that is unmissable. This was the playwright's last residence and his study remains exactly as he left it, as does the bedroom where he uttered his famously enigmatic last words, 'Tvert imot!' ('To the contrary!'), before dying on 23 May 1906.
Rooms have been restored and refurnished but the place feels totally and genuinely of its era. The guides are excellent, beautifully conjuring both Ibsen and wife Suzannah's daily life as well as the Oslo of the era.