Ross Ford and Andy Kelly added further tries for Edinburgh, who recorded their second pool win over the French side.
BBC: Edinburgh 32-14 Castres
2.
Kelly was delighted with his side's response after their 22-20 defeat to Tonga in their opening pool game last week.
BBC: Kelly proud of 'fantastic' Irish
3.
Ireland coach Andy Kelly rated Fiji higher than fellow Pacific Islanders Tonga and Samoa, the other teams in his side's pool, and he said they thoroughly deserved their semi-final spot.