After two years out, the 28-year-old made a winning return to the RBS 6 Nations in the 53-27 demolition of Italy and then went on to help dispose of wooden spooners Wales 33-5 at the Stade de France.
Tries by Castaignede, Vincent Clerc and Frederic Michalak and 18 points from the boot of goalkicker Dimitri Yachvili helped France to a third-place finish in the RBS 6 Nations table after defeats to Ireland and England.
And Maso said the key element in France's revival has been the form of Castaignede.
"Thomas is back to his top level - this is very encouraging for him and for us ahead of the World Cup," he said.
"He is back from hell. This time last year we were not even sure whether he would ever regain his former level one day. This is very good."
He added: "The perfect match would have been to play the same first half as we did against Italy and the second half we played against Wales.
"The Welsh defended very well, but we had good phases of game in the second half."