Bernières peninsula is the first great loop of the Seine in Normandy. It is at the end of it, above the town of Andelys, that Château-Gaillard rises up.
The fortress was at the centre of a vast array barring the river and controlling the Vexin region plateaus: on the Seine, an landing stage of three rows of stakes to break up any naval attack, a wooden bridge and the manor of the Ile; at the outlet of the Gambon, the fortifications of the village of Couture (Le Petit Andely) and Le Vieil Andely, separated by a pond; finally two small separate forts ahead of the castle, Le Muret and Boutavant (the fortified edifice on the Ile de la Tour).
Le Muret was located on the Vexin plateau, two kilometres south-east, in the parish of Cléry. It was a single round tower, erected on a motte and surrounded by a circular ditch. The motte and ditch can still be seen today.
Boutavant, five kilometres upstream, was built on an island in the Seine in the territory of Tosny. This small fort was a quadrilateral form eight metres on each side, some faces of the walls of which can be still be seen from the river bank. A line of light defences was supported by it and barred the Seine peninsula between Tosny and Bernières. 
Ensuring the effectiveness of these defences required troops, which John Lackland neglected to send.
From the summer of 1203 king Philippe-Auguste took these advance fortifications without difficulty, thereby depriving the defenders of Château-Gaillard of any room for manoeuvre.

Bernard Beck

Bibliographie

- Eugène Viollet-le-Duc : article " Château ", in Dictionnaire raisonné de l'architecture française du XIe au XVIe siècles, t.3, Paris, 1854-68. 
- R. Quenedey : " Le Siège du Château-Gaillard en 1203-1204 ". Bulletin des Amis des Monuments rouennais, Rouen, 1913.