2D Scroller game level design tutorials?
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I've completed a 2d scroller (plane bombing stuff) and have "programmed" all the stuff that should go into it. Looking good, if I may say so myself 
Anyway, I've been so focused in getting the bits and bolts and design of the game working, and now that it's time for actually creating the levels with these components I find myself completely stumped and in need of some guidance to make this a FUN game.
So my question is, are there any good tutorials/guides on this subject for these types of games? I've been googling like crazy, but nothing so far hits the spot.
Otherwise, do you have any suggestions to keep in mind when creating levels, other than the obvious fact that the levels need to be increasingly more difficult?
Thanks!

Anyway, I've been so focused in getting the bits and bolts and design of the game working, and now that it's time for actually creating the levels with these components I find myself completely stumped and in need of some guidance to make this a FUN game.
So my question is, are there any good tutorials/guides on this subject for these types of games? I've been googling like crazy, but nothing so far hits the spot.
Otherwise, do you have any suggestions to keep in mind when creating levels, other than the obvious fact that the levels need to be increasingly more difficult?
Thanks!