Jamie Heaslip (pictured above) counted Leinster's "grit" as the main reason they made it two wins out of two in Pool 2.
For the second season, they had to go behind early on to show what they were made of, Heaslip jamming his fist in frustration as the Castres maul made its' way over the line.
It wasn't ideal. Far from it.
"It doesn't help when we gifted them a try in the first two minutes," he said.
It wasn't the only period in which Leinster struggled as full-back Geoffrey Palis kicked penalties in the 42nd, 53rd and 56th minutes to send the home side 116-9 ahead. "Again, we gifted them nine points there at the start of the second-half.
"Mistakes again were killing us, not too dissimilar from last week. The good side of that is we've got a lot of work-ons," said the workaholic number eight.
The removal of Sean O'Brien and Cian Healy from the frame has meant increased carrying duties for Heaslip, hooker Sean Cronin and flanker Rhys Ruddock.
"We try and play a high tempo game, especially once we get thirty-twenty two metres from the opposition try line," he added.
"It's just all about work rate and trying to get around the corner, regardless of who gets their hands on the ball.
"It's just really running hard and making them tackle us, making them commit numbers to rucks, so we can exploit space for our backs."
There is never any doubt about Heaslip's appetite for work. He carried the ball 19 times yesterday.
It was eight more than the two next best Ruddock and Castres left wing Remi Grosso in what was a real war of attrition.
"I wouldn't say a lot of those carries were more than a metre or two. Some were probably minus metres. I can't really gloat too much about it.
"It's great to get the hands on the ball. But, I will do whatever is good for the team, be that rucking, tackling, poaching, carrying."
In the end, Leinster were content to take four points, if not the four tries, as they relied exclusively on the goal-kicking of Ian Madigan to reward their solid work.
Castres were no soft touch at home. They brought an intensity of focus in defence that stifled Leinster's attacking game.
"We are obviously delighted to come to Castres and get the win and go home with the four points in the bag.
"But, we didn't exploit the game. I thought we got momentum, at times, and broke them down, but mistakes killed us and we turned over the ball quite easily to them.
"We didn't help ourselves in that first 15 minutes. We compounded a lot of errors to let them back into the game and get some easy penalties. But, there was a lot of grit there".
For all that, Leinster know only too well from personal and recent experience how difficult it is to come away from Stade Pierre Antoine with a victory.
"That's a very happy changing room. It's a tired changing room," said Heaslip.
"It just shows that when we need to go to that place of really pushing it and grinding it out that we can do it.
"We're happy. Two wins out of two in the first two rounds. We would have taken that at the start of the competition."