Tales was at the heart of the French club's first European win since December as they bounced back from a crushing reverse at Ulster in Round 1.
Remy walked off with the Heineken Man of the Match Award at the Stade Ernest Wallon as the two-try defeat of the Aviva Premiership leaders blew the group wide open.
"We really wanted to prove that we can exist in this competition," said Tales.
"We had a bad start at Ulster but we had a good reaction tonight."
Just as he did at Ravenhill against group leaders Ulster, the No10 delivered a clever cross-kick with his right foot for a try and, just for good measure, dropped a long-distance goal that kept the Saints at arm's length.
"Yes, they were just coming back into the game and I'm glad I scored it in an important moment of the match," he added.
"We want to play a role in this group and we are just looking ahead to the next game. We would like to stay unbeaten at home and now we have new ambitions."
Castres clash with the pool's basement side Glasgow Warriors in Rounds 3 and 4, with the first tie taking place on Scottish soil on December 7 and the return being held back at Castre's home ground, the Stade Pierre Antoine, eight days later.