Never nice to see a player having to retire. But as psychic duck says, Castle was a poor player who shouldn't have been recruited in the first place. There must have been better options available? Trawl the 1st devision in England and I'm sure there are some decent scrummaging props who would jump at the chance of a pro contract.
Dragons current recruitment seems to be better though (if you overlook Tuilagi). Chavanga is more in line with the calibre of player they should be bringing in.
Ultimately, this means that the Dragons will need to actively recruit a replacement, hopefully it's someone who can stabalise their scrum and give Faletau the kind of platform he deserves. He does amazingly well off the back of a retreating scrum, but he'd be doubly as effective given a stable platform, and even more again at the back of a dominant scrum.