Why destroy Laketown's bridge?

[Eönwë]

Originally Posted by Sauron the White

while the city was crowded and cramped not allowing him to land, it was big enough for him to manuever around - or perhaps small enough enabling him to walk over the structures

Do you know how strong Smaug is?

My armour is like tenfold shields, my teeth are swords, my claws spears, the shock of my tail a thunderbolt, my wings a hurricane, and my breath death!

Even if this is an exaggeration, he could easily take down buildings with his arms and tail. This would be easy for him. He just walks around, smashing down buildings and killing all people in his way.

Originally Posted by Sauron the White

Closer combat means his eyes are closer to attackers and how does he protect those?

He had eye-caps like snakes. If Melkor designed a killing machine based on a reptile, wouldn't he have given it the best weapons protection reptiles had to offer? In fact, wouldn't he have give it the best weapons and protection any animal has to offer (And beyond)?

And as for your question of how they broke down the bridges?

What about Motte and bailey castles ages ago (around 1066+). They could get rid of that bridge immediately. (examples here, here, here, here and here
Or they could end it with a drawbridge, like here and here