Les Andelys is about 100km from Paris and 40km from Rouen. By car it is the most practical approach: take the A13 motorway from Paris toward Rouen, exit at Gaillon, and follow the D316 along the Seine. The drive from Paris takes about 1h20 in light traffic.
By public transport, the nearest SNCF station is Vernon (on the Paris–Rouen line), from where you will need a taxi or bike. Some travellers arrive by river: the Seine is navigable for private boats and canal barges, and there is a small quay at Petit-Andely.