The man made his way to the fortress with a friend to watch the sunrise after a night spent partying.
As the new day broke the two men lay down to sleep on a wall on the south-western side of the 13th century fortress.
At some point during the night one of the men tumbled almost 20 metres to the ground.
On finding his friend lying at the base of the wall, the other man called emergency services at 9.20am.
The injured man was taken to Sahlgrenska Hospital in Gothenburg. According to police, the fall resulted in injuries to his back as well as possible internal damage.