Ansbro was in scintillating form on the wing for Saints
Northampton secured a home Heineken Cup quarter-final with an assured win over Castres in their final Pool A clash.
Joe Ansbro and Phil Dowson crossed, with a Pierre Bernard try keeping Castres in touch at 15-7 at the break.
Saints soaked up some intense Castres pressure before Ben Foden finished off a superb Ansbro break to make it 20-7.
Sebastien Tillous-Borde touched down a consolation for Castres as Northampton, already through as group winners, guaranteed last-eight home advantage.
The Premiership outfit, however, will have to be granted special dispensation for the tie to be played at their 13,591-capacity Franklin's Gardens as European Rugby Cup regulations stipulate that knock-out stage venues must be able to accommodate a crowd of at least 15,000.
It is believed, therefore, that stadium:mk - the home of Football League side Milton Keynes Dons - is likely to stage the match.
Saints began their task with real purpose, with centre James Downey and fly-half Stephen Myler controlling the early ebb and flow from midfield, despite two worrying line-out steals by Castres.
And the visitors' early pressure paid off in the 11th minute when Myler whipped the ball wide before quick hands from Jon Clarke allowed winger Ansbro, starting in place of the injured Chris Ashton, to step inside and force his way over, despite the best efforts of full-back Florian Denos.
Myler narrowly missed the extras but Castres responded immediately only for the video referee to rule the ball had not been grounded as they attempted to drive over.
Saints were dominant in the scrum, though, and in the 25th minute that superiority yielded a drop-goal chance for Myler, who popped over with minimum fuss.
Moments later, neat footwork and an equally deft offload from number eight Roger Wilson sent Ansbro away again - the Scotland international making a good 30 metres from halfway before eventually being hauled down.
Support was quick to arrive, though, as Myler unselfishly laid off for flanker Dowson to cross with ease as Myler this time converted.
Castres had rested key players including Romain Teulet and France winger Marc Andreu ahead of an important Top 14 clash against Stade Francais on Thursday.
But although they had no chance of reaching the quarter-finals, the XV that were selected seemed determined not to lose the club's nine-month unbeaten record at Stade Pierre-Antoine.
In the 27th minute, Castres smartly switched the play from left to right before some quick recycled ball in the corner allowed Bernard to power over.
Full-back Bernard, deputising at fly-half, slotted over the two points to close within eight points of Saints.
Castres emerged with a spring in their step after the interval, and their determination to win the game was illustrated when they spurned a kick at goal inside Saints' 22 to instead kick to touch.
Saints' defence held firm, however, during the resulting onslaught for the line before poor handling from Jannie Borman relieved the pressure.
If Northampton's resilience was impressive, their innovation on the counter moments later was even more admirable.
After attacking a ruck with vigour and turning over, a Dylan Hartley reverse pass close to the touchline was picked up brilliantly by Ansbro, who surged for the line.
Ansbro's progress was halted by a Yoann Audrin tap-tackle, but Foden collected the pop-up to race over.
Myler was again wayward off the boot but he killed the game off in the 68th minute when Foden brilliantly collected an up-and-under to force an infringement before Myler's kick sailed over off the tee from 40m out.
Castres' commitment was rewarded in the dying moments when Tillous-Borde went over in the corner, however the score was purely academic as Northampton closed out the game to ensure they became the first English team to record a 100% pool record since Bath in 2002.
More importantly, of course, the result sealed a top-two seeding, with only a Toulouse victory away to Wasps on Sunday capable of displacing Saints as number one seeds.
Northampton head coach Jim Mallinder told BBC Radio Northampton:
"They were tough to beat. They wanted to win that game.
"We scored two good tries in the first half. Second half, they put us under a lot of pressure defensively but we finally turned them over, went the length of the field for a cracking try and that really sealed the game for us.
"I think that was the killer moment. It was fantastic for us and demoralising for them."
Northampton captain Dylan Hartley:
"We scored a couple of good tries but what won the game was the defence. We're really happy with the defence.
"Winning the pool and six from six is important. What we learned from last year is that going away in quarter-final rugby is certainly not easy so we're certainly happy with the home draw.
"There's no team we want to avoid. Everyone's going to be tough. The pool stages are always tough. Whoever has gone through has earned it and it's going to be tough when we come to that. Hopefully we'll get a good result."
Replacements: Martial for Audrin (62), Bai for Garcia (70), Peikrishvili for Coetzee (60), Giorgadze for Kayser (53), Ducalcon for Saayman (62), Adamou for Masoe (53), Tekori for Malonga (79). Not Used: Albouy.
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