"They're a good side and very powerful," he told Sky Sports. "It would have been nice to score four tries, but they defended really well and struggled to break them down.
"We spoke about getting ahead, but Castres stayed with us. I wasn't surprised by that and you've only got to look at their results, they're always very competitive.
"So I'm happy to get the win."
Saints have crossed paths with Castres in each of the past three Heineken Cup campaigns. All of those matches have been different, and this clash was certainly more similar to that of October 2010 to the romp of 12 months ago.
When Castres have something to play for they can be an obstinate, dogged and determined team. And they certainly had something to play for at Franklin's Gardens, coming into Northampton two points ahead of the Saints in the Pool standings and in the knowledge that a win would have knocked the home team out of Europe completely.
And when Rory Kockott pounced on a loose ball at the base of a Saints scrum to set in motion an attack that threatened the home line in the opening salvos, the Gardens was made very aware that Castres were in town to play.
There was a Marcel Garvey break, and a couple of overlaps created, but on each occasion Saints managed to cover back in time, and it fell to Kockott - at the second time of asking - to open the scores with a 16th-minute penalty.
It set the tone for the match. Both teams put some decent attacks together, but the approach play did not yield the tries either team needed to put the other under pressure.
Castres' play with the ball in hand may have been threatening early on, but their set piece was being taken to pieces by the Saints, with two Courtney Lawes steals just the start of a man of the match performance from the England lock. Lawes was also on top form defensively, too, making a try saving tackle on Romain Cabannes in the first half and putting a second half hit on fly half Daniel Kirkpatrick that took the Kiwi out of the game.
For their own part Saints were intent on putting Castres under pressure with a good kick and chase game, and James Wilson showed little rust from his time on the sidelines with an early follow up of a Lee Dickson box kick that put Saints on the front foot.
But while Saints kicked their way out of their own danger zone, when in Castres territory they put the ball through the hands, no more so than when Samu Manoa combined with Lawes to take Saints up the left wing deep into the visitors' 22. Manoa was superb with ball in hand, especially in the first half, and Soane Tonga'uiha caught the eye as well with one break from the base of a ruck.
Castres rode their luck, too, having one chargedown rebound into French hands and seeing George Pisi leave the ball behind in a dangerous Saints attack.
Nevertheless, there can be no doubting that whenever Saints got within sight of the line Castres managed to step up and make the crucial play to prevent the hosts crossing the whitewash.
Sometimes this was illegal - such as when Anton Peikrishvili pulled down a maul to be sent to the naughty step for 10 minutes - but sometimes it was just good play, not least when Joe Tekori wrapped up a maul a couple of minutes later to win his side a scrum five metres out from their own line.
So the hosts had to content themselves with a pair of Stephen Myler penalties which, while scant reward for his team's dominance, meant that when the teams headed for their half-time chat the scores were tied at six apiece.
While Saints started the second half well, Myler dropping the kick off onto the 10-metre line for his forwards to set up a decent position, the first shot at the sticks came from a missed Kirkpatrick drop-goal.
This was poor from the fly half, and Castres' tactical kicking was awry, too, with both Kockott and Garvey sending kicks straight into touch. The latter was costly, as there was no one home and had the speedster managed to keep the ball in play there would have been a half-decent chance of a score.
Instead it was Saints who took control on the scoreboard, Myler landing his third penalty and Ryan Lamb his first to establish a six-point lead going into the final quarter.
The hosts also had their best try opportunity during this period, Dylan Hartley being held up over the line after Calum Clark had charged down a kick for Phil Dowson to sprint up the left wing, but as with the first half Castres managed to nullify not just this, but also a couple of dangerous lineouts, Jannie Bornman in particular snaffling one ball against the throw 10 metres from his own line.
Kockott pulled three points back, but while Lamb's first attempt had dropped just over the crossbar his next two shots were more straightforward and gave Saints the breathing space they needed.
There was a heart-in-the-mouth moment when Cabannes just failed to hold an interception that would have put the centre well in the clear, but when Lamb's third shot a couple of minutes from time sailed between the uprights the win was sealed, making Kockott's last-gasp success purely academic.
10mins CASTRES Miss pen Kockott
16mins CASTRES Pen Kockott 0-3
19mins SAINTS Pen Myler 3-3
30mins SAINTS Pen Myler 6-3
33mins CASTRES Pen Kockott 6-6
35mins CASTRES Yellow card Peikrishvili
39mins CASTRES Miss DG Kockott
HALF-TIME SAINTS 6 CASTRES 6
41mins CASTRES Miss DG Kirkpatrick
50mins SAINTS Pen Myler 9-6
65mins SAINTS Pen Lamb 12-6
68mins CASTRES Pen Kockott 12-9
71mins SAINTS Pen Lamb 15-9
78mins SAINTS Pen Lamb 18-9
80mins CASTRES Pen Kockott 18-12
FULL-TIME SAINTS 18 CASTRES 12