The notable action came in the final quarter of the match as the Saints scored five tries to emphasise their superiority over a very poor French side. Stephen Myler kicked the remaining 20 points for Northampton.
With both teams already seemingly out of the competition, the game lacked real bite as two under-strength sides struggled to put in a performance.
There was a muted atmosphere as the crowd accepted there was little resting on the game, with the Saints frustrated by Castres' limited game plan.
The French slowed the game down at every opportunity and were content to allow their outside-half Pierre Bernard to kick away what limited possession they gained.
Northampton should have capitalised on their huge share of possession early on, but it was not until late in the game that they were able to do so.
The home side missed some of their key defence breakers in Chris Ashton, Roger Wilson and Lee Dickson, none of whom started, but until that late burst they lacked both the creativity and precision to effectively pierce the visitors' rearguard.
It was therefore left to Courtney Lawes to raise the tempo and it was his line-out work and crunching tackles which enlivened the early proceedings.
Castres began with only two of the 15 who started last week's game in France and they were soon under pressure. Northampton decimated the visitors' first two scrums.
It was Castres who had the first opportunity for points, only for Bernard to fire his 45-metre penalty attempt wide.
Stephen Myler took his first penalty chance to give Northampton the lead after 12 minutes.
Against the run of play Castres should have taken the lead. A clever chip over the top of the home defence from Bernard saw Pierre-Manual Garcia collect the ball, and had the centre passed then a try seemed a probability but he chose not to do so and the clear chance was lost.
Castres suffered a blow when Pierre Roussel was yellow-carded for pulling back Myler as the outside-half chased a kick ahead in the French 22.
But despite the Saints having nearly all the play in the flanker's 10-minute absence they could not add to their score.
Roussel returned, and Northampton continued to have the better of possession and territory but failed to take advantage and half-time came with them still only 3-0 ahead.
The Castres scrum continued to be frequently penalised by referee George Clancy and from two of the awards Myler extended the Saints lead with successful kicks from 35 and 45 metres.
A fourth penalty from Myler after 50 minutes gave the Saints a 12-0 lead before Bernard missed another opportunity for the visitors.
Dickson and Soane Tonga'uiha were introduced by Northampton on the hour in an attempt to give the Saints some attacking variety and it soon paid dividends when Ben Foden and Vasily Artemyev both outflanked the visitors' defence for tries which Myler converted.
Castres were demoralised and battered from their tackling stint and conceded further tries to Mike Haywood and Greig Tonks and a final one from Foden who ran 50 metres to score. Myler converted two of them.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.