The French side bounced back from their opening defeat in Ulster by outplaying their Aviva Premiership visitors for the majority of the match at Stade Ernest Wallon, and maybe should have won by a bigger margin in the end.
Paul Bonnefond got them off to a great start when he dived in for an eighth-minute try, after winger Romain Martial had caught Remi Tales' high kick across the field from left to right before off-loading the ball to his arriving centre.
Romain Teulet kicked the conversion and then added a 15th minute penalty just before Northampton finally got on the board with a Stephen Myler penalty kick of his own just one minute later to trim the lead down to 10-3.
That was then 10-6 with Myler's second penalty, but Castres were still the aggressors and Saints were being forced to defend manfully in the face of waves of French attacking, which only yielded a long-range Rory Kockott penalty eight minutes before the interval.
An uphill task got an even bigger incline when Northampton had South African prop Brian Mujati yellow carded five minutes after half time with the visitors under more pressure, and that strain started to tell as their scrum began wobbling just five yards from their own line.
Just when it looked like a penalty try could be awarded, Antonie Claasen peeled off the back of a rotating scrum and dived over the line to make it 18-6, which is how it stayed as Kockott missed the conversion attempt.
Saints survived with 14 men without conceding again, and they grabbed a lifeline when George Pisi kicked the ball forward off the floor after Kockott fumbled before grounding the ball under the posts - and Myler's conversion made it 18-13.
That gap was increased again to eight points when Tales slotted a drop-goal just two minutes later, but Myler's penalty on 72 minutes gave Northampton some hope of getting something out of the game.
Saints chanced their arm late on but it was not to be and they settled for a losing bonus point which had looked out of reach for most of the game after being second best for the majority of their encounter.
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