Jean Valjean had the singularity that he might be said to be doubly endowed, on the one side with the aspirations of a saint, on the other with the formidable talents of a criminal. He could draw on either as the case required.
This is not the first time we’ve seen Valjean do some acrobatic climbing and I don’t think it will be the last time.
Looking at the wall before him, he realizes that he could climb all 18 feet and make it over, but realizes that there is no way he could do it with Cosette on his back or hanging onto him in any toher way. It’s at this point tha the has a creative flash of brilliance - he can get “rope” from the cord used to raise the gas lamps on the street - the lamps that are currently in place but not lit!
Acquiring said cord, he fashions a harness for Cosette and proceeds to scale the wall, telling her to keep her back to it and stand still for him. Upon reaching the top he hauls Cosette up and then slides onto a roof on the other side that adjoins this wall. The ground is significantly lower on this side, but the roof is sloped and there is a lime tree that he can make use of, so he quickly moves across the roof and uses the lime tree to safely lower himself and Cosette to the ground.
The immediate danger has passed but they are in no way free and clear. Where are they? Were they seen? Will they be captured? We must read ahead to find out!