At nearby Cantelau, on the hill just west of the City, there is a viewpoint with explanations of what you can see as you overlook the old Roman city of Rouen.
The original city is north of the river and contains many buildings of ancient and historic interest. The allies tried their damnedest to blow the whole place up toward the end of the last war because the occupying Germans refused to go home. I don't suppose they bothered to ask what the natural inhabitants thought of it all. However, the old city has been painstakingly restored and is a perfect example of what can be achieved with loads of money.
The old cathedral is magnificent if you are into enormous religious edifices and the Place Notre Dame contains a great variety of varying architectural styles. The Gros Horloge (fat clock) is worth a looking at, even if they did move it from its proper place, and so is the public square where Joan of Arc was burned alive.
The new cathedral in the old market place commemorates the events surrounding her capture, betrayal, illegal trial, forced confession and subsequent execution (the West-Midlands Police Force obviously learned a great deal from the Catholic Church - allegedly). The museum opposite graphically describes her life and prolonged ordeal in several languages.
Across Pont Jeanne d'Arc on the south bank, the new town contains a gigantic new administrative centre and shopping precinct behind which is the easily-found Youth Hostel (Auberge de Jeunesse) situated on the inner ring road (118 Boulevard de l'Europe) next door to the Leisure Centre.