|Dragons (24) 31|
|Tries: Amos, Cudd, Evans, Hewitt Cons: Tovey 4 Pens: Tovey|
|Castres (10) 18|
|Tries: Smith, Samson Cons: Palis Pens: Palis|
Newport Gwent Dragons maintained their lead in Pool Two of the Challenge Cup with a bonus-point win over Castres.
Hallam Amos crossed less than a minute into his comeback, while tries from Nic Cudd and Lewis Evans gave the Dragons a 24-10 half-time lead.
The home side found it hard going despite a 58th minute red card for Alexandre Bias, before Ashton Hewitt scored the vital fourth try.
Dragons stay two points clear of Sale Sharks, who won 27-3 at Pau.
The top two face each other in the final round of matches on 21 January at Sale's AJ Bell Stadium, with the five pool winners and three best-placed runners-up qualifying for the quarter-finals.
The Dragons showed their intent from the start as a Castres knock-on allowed Sarel Pretorius to start a counter-attack and Jason Tovey to work Amos clear.
Cudd then smuggled his way over from a driving line-out maul and Evans crossed from close range. Tovey converted all three touchdowns.
David Smith shot over in the final minute of the first half for a converted score that seemed to turn the tide in favour of the French.
Lock Christophe Samson charged in after 50 minutes after some snappy handling and a Geoffrey Palis penalty on 56 had the home side rocking at 24-18.
Two minutes later, Castres were let down by their discipline as replacement lock Bias put in a reckless head and shoulder charge on Phil Price, and referee Peter Fitzgibbon brought out a red card.
French woes deepened as replacement prop Eric Sione saw yellow for a clumsy challenge, but the Dragons laboured to press home their advantage.
Eight minutes from time they finally cracked the Castres defence, with a Taulupe Faletau charge producing the chance for Hewitt to grab the loose ball for the crucial fourth.
Tovey converted for an 11-point haul to go with his man-of-the-match award.
Newport Gwent Dragons head coach Kingsley Jones told BBC Radio Wales:
"We wanted to play with ambition and not kick the ball too much.
"We had them under pressure and played good rugby. Our errors in the second half were 60% and we can't make that many errors, it was poor.
"We're still top of the group and hopefully we'll go to Sale on a positive note and have a go there."
Dragons: Meyer; Hewitt, Hughes, Warren, Amos; Tovey, Pretorius; Price, Dee, Harris, Hill, Landman (capt), Evans, Cudd, Faletau.
Replacements: T Rhys Thomas, Stankovich, Knight, Screech, Jackson, Davies, O'Brien, Scott.
Castres: Palis; Sivivatu, Combezou, Cabannes (capt), Smith; Fontaine, Seron; Diarra, Babillot, Caballero, Desroche, Samson, Martinez, Rallier, Taumoepeau.
Replacements: Sione, Beziat, Wihongi, Hannoyer, Bias, Wulf, Urdapilleta, Lamerat.