Been working towards this near-end goal for almost 3 months now.
The ability to generate regular and inverted terrain to generate tunnel sequences.
There will be some 'rock' based tunnels and some in the enemy base (with the black textures seen here).
Created special inverted elements to reside on the top sections (with more to come).
This was one of the last technical hurdles I had left - what remains is to be able to flip the inverted terrain on/off at the appropriate time (I need/want to keep it running parallel at all times so that it matches it's content properly with the non-inverted section).
I've got a list of other to-dos (including bosses) - but it's almost time to start optimizing and tuning so that I can re-start testing on mobile devices.
I think that I am going to add keyboard support for moving (wasd) for a web/pc version as well.
