I like the concept a lot, however:
1 Controlling the guy just isn't fun. There isn't a need for it, all I care about is whether or not my bridge works. Maybe an option for him to auto-walk?
2 It's extremely frustrating not being able to drag pieces you have already set. It's not gamebreaking, but it takes away from the enjoyment having to constantly delete pieces to make minute adjustments.
I like this game, but it's hard to tell where the bullets are a lot of times. They are small and pale in color and they aren't very distinguishable from the background. Maybe increase the color saturation of the bullets?
Also, the numbers after collecting gems are distracting and I'm not real fond of the collection mechanic in general.