What's the difference between a "personal boundary" and a "putting up a wall"?

I’ve come to see recently that I don’t truly know the difference between a boundary and a wall.

I want to be more approachable, but I don’t know how to do this very well. I always want to keep people at arm’s length until I can assess whether they are a threat to me, especially men. This basic lack of trust in people is almost certainly is an old childhood defence mechanism, which was utterly necessary then. Jellies who’ve been here long enough know that I had good reason to be afraid of people in general and men in particular as a child.

But it’s not helping me now, not with jobs, relationships, making new friends… I need to know the difference between a wall and a boundary so I can be more approachable. I want to assert myself confidently in the world, stop worrying that I’m about to be used or hurt and feel confident that I can get away from people who make my “Spidey senses” tingle in a “self-assured adult woman” way, and not a “frightened little girl” way.