The French outfit visit Franklin's Gardens on Friday night sitting two points ahead of the Saints in the Pool 4 standings, meaning that a win is essential for the home team to continue their European ambitions for at least another week.
But the Saints are in contention thanks to an against-the-odds win over pool leaders Ulster in December, and Mallinder says that the atmosphere in the camp this week has been similar to the days before the trip to Ravenhill.
"We still have a massive point to prove in Europe, and the players know what is expected of them on Friday night," he 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."
Tickets remain on sale for this massive Heineken Cup clash - to order just CLICK HERE and follow the straightforward instructions, or call 01604 581000 before 7pm this evening (Thursday) or from 9am on Friday.
NORTHAMPTON SAINTS TEAM TO PLAY CASTRES
Heineken Cup Round 5
Franklin's Gardens
Friday, January 11th, 2013
Kick off 8:00pm
15 Ben FODEN
14 James WILSON
13 George PISI
12 Dom WALDOUCK
11 Jamie ELLIOTT
10 Stephen MYLER
9 Lee DICKSON
1 Soane TONGA'UIHA
2 Dylan HARTLEY (capt)
3 Paul DORAN JONES
4 Samu MANOA
6 Calum CLARK
7 Tom WOOD
Replacements
16 Mike HAYWOOD
17 Alex WALLER
18 Tom MERCEY
19 Mark SORENSON
20 GJ VAN VELZE
21 Martin ROBERTS
22 Ryan LAMB
23 Tom MAY
