1. It “lies near the mouth of the Somme before the flat coastline soars up from the beaches of southern Picardy to the Brittany cliffs which run on to Le Havre”. (CR5)
This information yields little other than Royale is near the mouth of the Somme and the coastline is flat. The Brittany cliffs don’t run on to Le Havre from Picardy – Brittany follows Le Havre from Picardy.
2. “But as Deauville killed Trouville, so, after a long period of decline, did Le Touquet kill Royale”. (CR5)
One can conclude that Royale is a similar distance from Le Touquet as Deauville is from Trouville.
3. “Le Touquet would have to yield up some of the treasure stolen over the years from its parent plage”. (CR5)
Again this implies that Le Touquet and Royale-les-Eaux are neighbours.
4. The “longest beach in Northern France”. (OHMSS1)
Le Touquet has a long beach, although we’ve been unable to determine how long.
5. Bond is driving on “that fast but dull stretch of N1 between Abbeville and Montreuil”. He reaches a sign saying “Montreuil 5, Royale-les-Eaux 10, Le Touquet-Paris-Plage 15”. (OHMSS2)
6. He then follows the road through Montreuil and over the Étaples-Paris level crossing, after which “the left-hand turn for Royale came up”. (OHMSS2)
The left hand turn actually leads to Le Touquet, although it is 16 km from Montreuil. 10 km south of Le Touquet is Berck-Sur-Mer. This has both casino and a beach, two attributes essential for Royale-les-Eaux. Although Royale may not be based directly on Berck, it is certainly approximately where Royale-les-Eaux would be from the evidence presented by Ian Fleming.