When you're ready, you can click the green Start button to send your little critter tottering trustingly out onto your bridge. If the bridge snaps, you'll have to try again. You can choose to Undo a single move, Reset the whole level, or add more support to the existing design, if you have any funds left. There's no real penalty for failing, so you can try over and over again as often as you like. In fact, you'll probably want to make use of the "Show Stress" button at the top of the screen, which will highlight areas that are under particular strain when the Euwin tries to cross and help you figure out where the problem lies. That the levels take place on a grid makes it that much easier to really fine-tune your measurements.
For those of us with less than a passing interest in architecture, Bridgecraft can prove frustratingly challenging at times, as you watch construct after construct surrender to the elements and send your unhappy looking Euwins tumbling into the watery depths. You may not need a degree in engineering for this one, but it helps to have a basic understanding of structure and support. 70 levels is plenty of time to become a building master, but I'm still not sure whether the physics engine is unforgiving, or I'm just inept at engineering. I'm sorry, little Euwins. Your trusting eyes will haunt me in my dreams!