[alatar]

I'd like to thank whomever posited the observation that Smaug may not have wanted to attack the city entirely from the air, but as there was no bridge or boat to get him to the city, he was forced into plan B, namely to burn the city and hope that the treasures therein would somehow be salvageable. He may even have let some persons go, as they would do the hauling for him for cheap, and when the treasures reached the shore, later he could gather them up along with a snack or two.

Anyway, I finally figured out how the bridge was brought down, and had to look no further than LotR. What aquatic creature was typically found living outside dwelling places which existed that could rip down large wooden structures in seconds? Why, it's the Watcher in the Water!

Why couldn't the Laketown persons, like the Morian orcs, have entered into contract with a cousin of said creature? It could feed on any garbage or other stuff dropped down to it, and in return it would guard the bridge. Surely you didn't think that Bard and a few stout fellows were all that protected the town?

When Smaug appeared, the creature could have easily thrown down the bridge and then hid in the safety of the deep water, 'cleaning up' those Laketown persons that were 'sinkers.'