10th prisoner kingdom 2

The walk through says to destroy the bridge to get to the prisoner. Which bridge? I have tried to destroy the main bridge and the bridges that I have built and nothing seems to work. Any one got any other ideas. this is the last quest for 2nd kingdom because the last tiara is there also. HELP please.

you have to destroy the bridge that keeps you from walking through the water to get to the prisoner.

How do you destroy the bridge?