I read that the seven Euro notes feature designs from the seven european ages, baroque, renaissance, mediaeval, modernist, etc.
The front features construction from the various countries, ours are the two windows on the 20 Euro note, which are from some one of the monastries or abbeys. The rear of the notes feature bridges from around europe. Ochoa mentioned the Portuguese one, the Pont du Gard is the large viaduct shown on the back of another. Maybe others could identify the rest of the buildings and bridges?