The game's significance is not lost on Saints rugby director Jim Mallinder after Northampton gave themselves a Pool Four lifeline with victory over group favourites Ulster in Belfast last time out. And while they still have it all to do, currently standing two points behind Castres and five adrift of Ulster, a quarter-final place could still be achieved.
"We still have a massive point to prove in Europe and the players know what is expected of them on Friday night," Mallinder said.
"We need a win to keep our destiny in our own hands, especially as Castres are two points ahead of us in the pool.
"We also know that it will not be easy against one of the top teams in France. Castres have been in superb form over the past few months and have been playing a style of rugby which makes the most of their big forwards and speed out wide.
"It is a massive challenge for us and all of the focus has been on what we need to do on Friday. We are just about playing knockout rugby, so getting the win is all that matters.
"But we've been playing well recently, scoring good tries and defending well, so there is plenty for us to build on and we are in a positive frame of mind to get our home form back on track."
Mallinder has made three changes from the side that accounted for Aviva Premiership rivals Exeter last weekend, with wing James Wilson replacing Ken Pisi, lock Samu Manoa taking over from Mark Sorenson and England flanker Tom Wood returning to back-row duty.
Ulster are also in action on Friday, entertaining a Glasgow outfit that has collected just two points from four Heineken Cup games so far this season.