The Drumlish Trade Post needs a special deed to unlock. This Trade: Dyed In The Wool Drumlish Trade Post Deed Location Guide For AC Valhalla will tell you how to track down the deed as the game just asks you to investigate the Trade Post, which doesn’t bring up any clues or other information.
Trade Posts play an important role in the Wrath of the Druid DLC, so it makes sense that you want to find all of the Trade Posts. Trade Posts open up new trade options that can unlock brand new armor sets and provide other rewards and goodies. There are also special quests that offer double the rewards if you have certain buildings constructed at specific Trade Posts. The Drumlish Trade Post is one such location, so you’ll want to find the Deed so you can upgrade the Trading Post as soon as possible.
Trade: Dyed In The Wool Drumlish Trade Post Deed Location Guide For AC Valhalla
There may be a clue or another tip in the Trade Post itself but I never found one. To find the deed you need to travel South to Knockfree. There’s a tower here with a golden chest near the top. You have to shoot through a grate to destroy an explosive jar, this in turn destroys the wall, revealing the path to the chest.
Right next to the chest is a skeleton. Loot the skeleton to claim the deed. You can now return to Drumlish and build the Trade Post.