Skipper Hartley was satisfied with his side emerging with a bonus point 24-15 victory over Glasgow Warriors in their Pool opener but admits the trip to France will be a different proposition.
The England international hooker fears a backlash from Castres after the French side's one-sided 41-17 defeat to last year's finalists Ulster in Belfast on the opening weekend of the competition.
"We said in the week leading up to the game, if you want to go well in the Heineken Cup you've got to win your home games," said Hartley
"Usually your first game is fairly important as well so we'll take a lot of confidence from that.
"But we've got to back it up in France because it's a huge challenge going away and winning over there, regardless of who you're playing."
Castres may have struggled against a rampant Ulster in Belfast but the French side have won 17 of their 26 European games on their home patch.
Northampton wing Ken Pisi made five clean breaks in the opening round of the Heineken Cup, more than any other player. However, he failed to convert any of them into tries.
Saints centre Dom Waldouck was the top tackler of round one, completing 19 tackles at a 100% success rate.
The Frenchmen have won just one of their last 10 Heineken Cup matches: 41-22 at home to opponents Northampton on 10 December 2011.
Northampton have lost only one of their last six clashes with Top 14 opponents: 22-41 at Castres last season.