Thank you very much for the review, riuthamus! It was instructive. 
Argh, I see now that my game calls for a tutorial--at a remove, and seeing someone else play, there's a leap of logic called for that isn't at all obvious. Suffice it to say that there's more to be found in the game (including the second theme) once that initial leap of logic is made. ^^;;;
For any who find themselves stuck after crossing the first chain:
Spoiler
The secret is to shoot the chains.
... Not while standing on them. That would be a bad idea.
Something else that seems to be inobvious:
Spoiler
You can hold down the fire button. Think old-school shooters. In fact, the collectible weapons use that feature for charging shots.
You didn't even get to climb, you just had to walk towards a wall (albeit one without a block). I guess I failed pretty hard on the tutorials, without naming the keys and giving a bit more info. A shame, but clearly to blame only onto myself. 
) 
