Romerberg is the central square in the Old Town (Altstadt) of Frankfurt. Probably the most visited sight in Frankfurt - it has a collection of interesting looking buildings surrounding the Square. Most of these buildings have been rebuilt though - as the original buildings were destroyed during the War.
There's an interesting history behind Romerberg. Since the 12th Century it has been the site for large trade fairs drawing visitors and merchants from nearby countries. It was the forebear of the Frankfurt Messe - the famous Frankfurt Trade Fair Grounds which is still being used for large global trade fairs such as the Frankfurt Book Fair, Frankfurt Motor Show, Furniture Fair, Light+Building,etc.
The Square is also a good place to try some local food as there are many shops and small cafes around. In the evenings it'll probably fill up with people enjoying a pint or 2, judging from the number of tables and chairs around.
I ended up trying a pork chop bun. It was served with a barbeque sauce and some lettuce. The bun was nice - soft and fluffy with a slightly crunchy crust. The pork chop was a bit tough and dry. Probably would have been better served warm.