Done all levels without sanbox. Really easy. The only thing that I hate, is that when one of the critters gets on the bridge, the bridge is prone to total destruction. I mean c'mon, they don't weigh tons or jump on it 'cause they're stupid... or are they?
Try to make a structure looking like this:
/|X|X|X|X|X|X|\ This offers maximum stability, even without needing a support point (except for the ends, of course)