I visited the Charles Bridge (Karluv Most) twice whilst I was in Prague. Firstly as the light was fading (which is about as close to romantic as I get) and again the next day to see how it looked in the sunshine.
On both occasions it was beautiful and very crowded.
The bridge was commissioned by Charles IV in 1357 and connects the Old Town with the Little Quarter. There are a number of baroque statues decorating the bridge, although some have rather gruesome histories.
I loved looking out at the view, watching (and trying to dodge) the people going by and seeing the artists at work. We were serenaded by a barbershop quartet, ate ice-cream and took a million pictures. Like these:
I was going to crop all of the crowds out of the pictures, but I think it works and it shows you how busy it was!