The Battle of Varaville was a fine example of William's ability to use Normandy's landscape to his advantage and seize opportunities when they arose. The combined armies of the French king and Count Geoffrey of Anjou would have been formidable. The very fact that they made it so far into Normandy is proof of this.
By letting the king's troops cross the River Dives, William was able to then emerge from behind the hills where his troops had been hiding and to overpower the army from Anjou. Stranded on the west bank by the rising tide of this river (Varaville is just two miles from the sea), the French king could do little other than watch.