Bridge Troll Guilds are guilds clubs, and unions of trolls found in several countries. Their main purpose is to work for rights for trolls in those countries.
The most well known guild is the Daventry Bridge Troll Guild. There may have been another guild in Llewdor but it was gone decades later when Waggle travelled to the country. She formed her own guild and charity there to 'feed the trolls'.
There is no Bridge Troll Guild in Avalon. But this seems to suggest that trolls may live in the great forest there.