I've decided to incorporate a platform game inside the idea of falling from many levels. For example you may start the game falling having to tilt your way through the tunnel when all of a sudden you reach gravity and land on the floor; this is when the platform style game takes place.
As well as just that to incorporate the tilting aspect still I thought about maybe guiding your Viking's weapon to kill the enemy in order to explore some more. Or maybe using a grappling hook to help you reach higher levels, to also help you explore. To make a change from free falling, maybe as the screen scrolls along, there's a tunnel at the end which delves deeper into the Underworld. Making you take control of the character again guiding him down the tunnel.