Northampton, with top spot in Pool One of the Heineken Cup secured, came here chasing the victory that would give them one of the top two seedings in the quarter-finals. Three tries, each featuring the electric pace of Joe Ansbro, ensured they did it, with a hefty chunk of defensive stoicism thrown in.
Franklin's Gardens is four hundred short of the 15,000 capacity demanded by the organisers to stage a last-eight tie, so a decision must be made over whether to switch to Milton Keynes or plead for a slight bending of the rules. "I will keep fighting to have it at home," said Jim Mallinder. "As a coach, and as players, that's what we want." By becoming the first English club since Bath in 2002 to win all six of their pool matches, the Saints may feel they have earned it. If the opponents turn out to be Leicester, the bigger venue may be inevitable.
"Last year we were just happy to get through," Mallinder said of the qualifying route that took Northampton away to Munster and no further. "We set our stall out this season that we wanted to improve and so far we have done that."
Castres were at half-strength, saving their perkiest XV for a Top 14 match against Stade Français. Though a record of 10 wins in 10 previous home matches was not be sniffed at, unlike the weather on a freezing night flecked with snowflakes, the first 10 minutes, culminating in Saints' first try, by Ansbro, suggested Northampton would get what they wanted. Tom Wood, a candidate to be called into England's Six Nations Championship squad on Monday, soared in the line-out; easy ground was made in contact. Rapid rucking and Jon Clarke's final pass helped send Ansbro over near the right corner.
Ansbro, a Scotland centre, was on the right wing in place of the injured Chris Ashton, while Calum Clark, normally a flanker, filled in at second row for Courtney Lawes. The efforts by Mallinder to strengthen his squad are paying off. Ashton should be fit to join England's training trip to Portugal this week.
Even a team more worried about their French league place will lick its lips at a bit of the old heave-ho near the opponents' line. Castres eschewed a kick at goal when Foden got isolated in his 22, and drove a line-out to within a gnat's whisker of scoring through Chris Masoe. The Northampton forwards did well to hold the former All Black flanker out.
When Myler calmly manoeuvred himself for a drop-goal after 24 minutes, Northampton led 8-0. Roughly 60 seconds later they had a second try. Myler lobbed the ball casually to Phil Dowson to score, after Ansbro had broken free from Roger Wilson's pass out of the tackle and Clark had given support. Myler converted.
A rare lapse by Clark and James Downey then allowed Castres' fly-half, Pierre Bernard, to score and convert a try for his side which hacked into Saints' lead in the 30th minute. But Castres' strong start to the second half foundered on Clark bucking up his ideas and his tackling and Foden at full-back following suit.
The last time Northampton had a home quarter-final, in 2000, they won the cup. It might not be wise to stick the mortgage on a repeat. There is the curious statistic that none of the seven previous teams to win throughout a pool have reached the final. Also, Toulouse and Leinster look better bets on all-round star quality.
For now, the Saints were content with Foden finishing the try of the night, in the 57th minute. Counter-rucking forced Castres back, Clark and Ansbro went up the right and a flip behind Dylan Hartley's back set Ansbro clear. A tap-tackle stopped him; Foden arrived at pace to see off a Castres straggler.
A ruck penalty allowed Myler to make it 23-7 after 69 minutes; Castres kept on booting their penalties to touch, trusting the elastic Uruguayan Rodrigo Capo Ortega. Goodness knows how, but Dowson prevented the No 8, Jan de Bruin Bornman, from going over before the scrum-half Sébastien Tillous-Border slid in at the corner. Bornman really should have scored. Northampton's rebirth as European contenders continues.
Castres: F Denos; M Nicolas, Y Audrin (R Martial, 61), P-M Garcia (S Bai, 72), V Inigo; P Bernard, S Tillous-Borde; M Coetzee (A Peikrishvili, 60), B Kayser (A Giorgadze, 52), D Saayman (L Ducalcon, 67), S Murray, R Capo Ortega, C Masoe (capt; G Adamou, 52), J Bornman, S Malonga (I Tekori, 80).
Northampton Saints: B Foden; J Ansbro, J Clarke, J Downey (B Reihana, 67), P Diggin; S Myler (S Geraghty, 78), R Powell (S Commins, 78); S Tonga'uiha (A Waller, 72), D Hartley (capt; B Sharman, 69), B Mujati (T Mercey, 72), C Clark, M Sorenson (C Day, 57), P Dowson, R Wilson, T Wood (M Easter, 67).
Referee: P Fitzgibbon (Ireland).