The Castle? You'll need to do a bit of work before you unlock it.
Once you've rescued Preston, keep doing Minuteman quests for him, and keep establishing new settlements. Eventually he'll give you a quest to retake the Castle.
Make sure you're ready for it, though. I won't spoil it, but it's a tough fight.
Edit: actually, you can unlock it whenever you want if you go there by yourself and clear it out. But last I heard, doing that could prevent it from being a Minuteman stronghold (possibly patched by now). And it's way better as a Minuteman stronghold. They clean it up, open, and have a good merchant.
(Edited 12 minutes later.)
I don't do that. The interface is a pain to work with.
@691,178 (D-g !jJEom8Ii3E)
Really? I'm about to get robot gorillas for mine.