Walls walls walls

So, throughout my 18 years of life, I've learnt a couple of things about myself.
Things change, but one thing about me that hasn't and aren't changing: my walls.
I have walls.
Not insane walls where nobody ever sees how I'm really feeling.
But walls none the less.
When people first meet me, they assume that I'm an innocent and demure individual.
I don't mind when people assume this, it's easier to be honest.
People don't question who you are when it seems like there isn't much to question.
The thing with people, especially the insecure ones, they look for your flaws and exploit them.
They judge you without really knowing you.
I'm a strong girl, but judgement is something I'm pretty lousy at handling.
So, I will try to avoid situations where I can be judged.
It's an automatic response in most situations, I trust that my sub conscience knows when I will be judged.
But this is a simple wall, everybody has this, right?

All year this friend of mine made me question my walls.
He makes me build walls, new walls, strong walls, constantly.
I don't understand why. or how.
He understands me better then most.
Which is why he constantly calls me out on my walls, because neither he or I understand their purpose.
I don't know what to do because as of lately, I cannot tear them down.
It's like having reinforced steel walls around your heart.
He doesn't get it and neither do I.
He is often flabbergasted by his inability to assume how I feel or what's on my mind.
I swear I don't do it on purpose.
I feel like I'm trying to be that innocent and shy version of myself that most people see.

But how on earth do you try and keep up a façade of who you are, when somebody already knows you??