Thanks for all the suggestions and comments.
Inspired by Cedrics comment, I've modified the game (just uploaded it) so that it can be played by up to 4 players simultaneously.
It now also gives you a score based on how quickly the level is solved.
I've also added a 7th (even more demanding) level and some startup-interface using the G4P library.
As for the general difficulty of the game: It becomes easier over time
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I have to know, bet who played the levels XXX times to make sure they work (and to nudge things around)
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But if people would like to help me designing new levels, just drop me a line (or PM)...
@pwn monkey: I've never heard of a game called "gravity hook", could you point me to the proper place? I don't claim to have "invented" this game though, I think the ideas are pretty basic and might have been used in many other, older games (?).
@yelele_eu: Thanks for the link to "string theory". Haven't heard of that one before either. I like it's simplicity. Very inspiring.