[alatar]

Maybe they were prepared for any type of land invader, and so designed the bridge to be 'thrown down' easily. This would be a nice security device.

And how much did they know of Smaug? From the histories that they may have learned from the elves, removing the bridge as a means of access may just have been prudent, as would have been helpful at Nargothrond when Glaurung came calling.

Was Smaug more birdlike or more like a helicopter? What I mean is that I don't see the old worm perching on the weather vane of the tallest building in Laketown. He may have needed some space to land, and that space may not have been available. Would he have come down anywhere, unsure whether the structure would hold him, not knowing if he were going to soon test his swimming skills?

Just some thoughts.