Keep in mind with bases, that you can set-up supply lines between them so all your stored items are accessible between them. Doing that, you can store all the stuff you've built and just place it again in the new settlement. I'm really happy with Sanctuary right now, but a castle sounds AMAZEBALLS.
There's no trader in Concord, but you can store your stuff in something, fast travel back to Sanctuary to offload it and then return to pick up the rest. 99% of what you pickup can be broken down into crafting elements so it's handy to grab everything (especially coffee cups... MAKE SURE YOU GET COFFEE CUPS)