The top side portal is here
The tunnel inside Bragernes Hill climbs in spiral-form from 50 m (approximately 650 feet) to over 200 m (approximately 2600 feet) above sea-level. You can literally put your steering-weel into "veer-left mode", and continue up and up until you reach he top. The tunnel, which is 1650 metres long, is formed like a coil spring, and makes six sharp turns on the ascent up to the top.
The Spiral Tunnel was opened on the occasion of the city’s 150 years anniversiry in 1961. It was originally an open quarry that was dug out at the turn of the century to supply fill for the city’s road construction work. The Spiral Tunnel has been a popular attraction ever since.