Let me walk you through it.
What bridges? Certain specific bridges? Or all the bridges we know of? Hmm. Okay, we do know know that the Romans built some bridges. They had the know-how, and their engineering was top-notch.
Now let's imagine we're standing on a riverbank, taking in the sights. We can see two bridges of unmistakably Roman design. Someone standing beside us questions Roman engineering.
Our response is:
The Romans did build those bridges.
Now let's change the scenario a little bit. Suppose that instead of having doubts about Roman ingenuity our companion should question the Roman's willingness to use it too far from home (we're in Germany right now). He or she says that while, say, Rome and Florence are awash with Roman stuff, very little of it can be found north of the Alps. Now it's time for you to point at the bridges and say:
Those bridges did get built by the Romans.
So, I guess, the trick is to introduce the word "get" somewhere in there.
I hope this helps.