Thought I'd add some detail from the development process. Here are a couple of images showing how the levels are constructed:

 

They are built in 3D to give the parallax effect of the backgrounds - although the camera is locked to a 2D plane, it's still doing all the usual perspective projection stuff.

You can also see the culling grids that I use to keep the processing of the backgrounds as efficient as possible. I haven't really had much feedback on performance - I'd be interested to know how the game runs on different devices.

The circles visible around the hooks are a guide for spacing grapple points - it's important to get this right because it has a big impact on the flow of the game. Not sure I got it right on every level.

All of this is output to a level xml file that the game loads.

When I came up with the design of the game, one of the interesting things for me was that you mostly start at the top of a level and finish at the bottom. This is opposite to how most games work but is a natural consequence of gravity being the driving force of movement through the level. I can't think of any other games that use this approach - I'm sure there are plenty, though - no such thing as an original idea, especially in the world of apps.