At that stage a record victory over the Italians looked on the cards, even more so when Castaignede increased the lead further when he touched down Les Bleus' sixth try, just 34 seconds after the restart.
But it was not to be as Italy, roared on by a faithful, if shell-shocked Rome crowd, hit back.
They scored four times in the final 41 minutes of the match through Mirco Bergamasco, Aaron Persico, Matthew Phillips and Ramiro Pez's try in first-half injury time.
Castaignede admitted a lack of concentration from his side had contributed to the turnaround in Italian fortunes.
"We controlled the game well in the first half but we cannot be happy with the tries we conceded in the second half," he said.
"We made a few mistakes after the pressure had already been achieved (in the first half).
"I think the Italians played much better in the second half and we didn't control the ball very well, so we deserve what we got.
"(But) I think we should be positive about this game, it was enjoyable to play in."