Question: Axel Nicolai Herlofson was imprisoned in which medieval castle near Oslo, Norway?

Context paragraphs:
Arendal crash: The Arendal crash ("Arendalskrakket") was an economic crisis in Arendal, Norway, that occurred when the bank Arendals Privatbank went bankrupt in 1886 and the bank's co-owner Axel Nicolai Herlofson was revealed to have defrauded customers and co-owners systemically.  The crash led to several other bankruptcies and unemployment in Southern Norway, and marked the end of Arendal as an important shipping town.  It took over 80 years to overcome the effects of the crash.  Herlofson was sentenced to six years penal labour.

Airds Castle: Airds Castle is a ruined medieval castle near Carradale, Kintyre, Argyll and Bute, Scotland.  The castle held a position on the summit of a rocky headland between Carradale harbour and the bay of Port Righ, looking across Kilbrannan Sound to the Isle of Arran.

Krásný Buk Castle: Krásný Buk Castle (German: "Schönbuche") is a ruined a medieval castle near the village of the same name (German: "Schönbüchel") in the borough of Krásná Lípa ("Schönlinde") in the Šluknov Hook in the Czech Republic.  Of the original castle only a few section of wall and parts of the circular rampart ("Ringwall") and castle moat have survived.

Oslo Hospital (station): Oslo Hospital is a light rail station on the Ekeberg Line of the Oslo Tramway.  It is located near Oslo Hospital in Ekeberg, in the borough of Gamlebyen, in Oslo, Norway.

Burg Kreuzenstein: Burg Kreuzenstein is a castle near Leobendorf in Lower Austria, Austria.  Burg Kreuzenstein is 265 m above sea level.  It was constructed on the remains of an early medieval castle that had fallen into disrepair and was then demolished during the Thirty Years' War.  Intended to be a family vault for the Wilczek family, it was rebuilt in the 19th century by Count Nepomuk Wilczek with money from the family's large Silesian coal mines.  Kreuzenstein is interesting in that it was constructed out of sections of medieval structures purchased by the family from all over Europe to form an authentic-looking castle.  Thus, the castle can be considered both a 'neo-' and 'original' medieval structure.

Akershus Fortress: Akershus Fortress (Norwegian: "Akershus Festning" ) or Akershus Castle (Norwegian: "Akershus slott" ) is a medieval castle that was built to protect Oslo, the capital of Norway.  It has also been used as a palace and as a prison.

Axel Nicolai Herlofson: Axel Nicolai Herlofson (29 August 1845 in Østre Moland – 1910 in Christiania) was a Norwegian fraudster who is known for causing the Arendal crash, after defrauding customers and co-owners of Arendals Privatbank which he had co-founded in 1874.  His scheme had some similarities to a Ponzi scheme, and he had systemically falsified the bank's accounts for several years to cover up being insolvent.  In 1886 the bank went bankrupt, and an investigation of Herlofson was initiated.  He attempted to flee the country, but was arrested in the last minute and sentenced to six years imprisonment with hard labour, which he served in Norway's strictest prison at Akershus Fortress.  He died in Christiania.

Mategriffon: Mategriffon or Mategrifon or Mathegriffon was a medieval castle near Messina, Sicily, initially built as a wooden castle by Richard Ist, king of England and demolished before his departure in 1191 from Messina for the conquest of Cyprus.  Another reference to the castle is also made during the Sicilian Vespers, as the place where Charles I of Naples's Vicar Herbert and his family safely stayed during the uprising until their safe departure was negotiated.

Heighley Castle: Heighley Castle (or Heleigh Castle) is a ruined medieval castle near Madeley, Staffordshire.  The castle was completed by the Audley family in 1233 and for over 300 years was one of their ancestral homes.  It was held for Charles I during the English Civil War and was destroyed by Parliamentary forces in the 1640s.  The ruinous remains comprise masonry fragments, mostly overgrown by vegetation.  The site is protected by Grade II listed building status and is a Scheduled Ancient Monument.  The castle is privately owned and is not open to visitors.

Fonmon Castle: Fonmon Castle (Welsh: "Castell Ffwl-y-mwn" ) is a fortified medieval castle near the village of Fonmon in the Vale of Glamorgan and a Grade I listed building.  With its origins rooted in the 12th century it is today seen as a great architectural rarity, as it is one of few buildings that was drastically remodeled in the 18th century, but not Gothicized.  The castle is believed to have remained under the ownership of just two families throughout its history; from Norman times the St Johns and from 1656 the descendants of Colonel Philip Jones.

Supporting paragraphs: Axel Nicolai Herlofson Akershus Fortress
Answer: Akershus Fortress