The Warriors were beaten 31-3 at Munster last weekend, but Townsend has retained faith in the majority of the line-up, with wing Tommy Seymour, fly-half Scott Wight and number eight Josh Strauss drafted in.
Sean Lamont is rested, while Duncan Weir and Ryan Wilson drop to the bench. Sean Maitland, who recently joined the Warriors from Canterbury Crusaders, is in line to make his debut off the bench at Scotstoun as Glasgow bid for a first point of their Pool Four campaign.
Townsend told www.glasgowwarriors.org: "We're keeping faith with the core group of players who were beaten by Munster, as we know that that level of performance was not a true reflection of that side.
"I expect to see a reaction and a determination from those players to prove that they deserve their place. We pride ourselves on being tough to beat and we didn't do that in Ireland.
"There has been a renewed determination this week as that was a difficult defeat to take.
"We've prepared very well this week and have some existing knowledge about Castres, having played them earlier this year in a pre-season match.
"They have clear strengths that they will want to bring out on Friday night, so we've worked hard at how best we can nullify these, while also looking at opportunities for us to win the game.
"They are on a fantastic run at the moment in the Top 14 and will be looking forward to carrying on that form in this competition. We know our record in the Heineken Cup is not as good as it should be and we're fighting to stay in the tournament.
"The next two games will determine how the rest of our season will go, and we'll be doing all we can to still be in contention in this competition."