You can only theoretically have boundaries between separate people. It's as though, if you acknowledge where you stop and other people start, or what is yours and what is theirs, ie the two of you are distinctly separate, then any blurring of that distinction is a crossing of boundaries.
So if someone comes along and 'steps on your toes' or violates your independent freedom, as it were, e.g. they blame something on you or disrespect 'you', then they've `crossed the line`. And if you are aware of where your 'line' is, then you can tell it's been crossed, and then its up to you to respond in a way that defends that boundary.
As separate human bodies it does at first seem natural and healthy to have healthy boundaries. Except that, even when you get healthy boundaries on an individual level, you still have to constantly maintain it. And that's because there's still a weakness. You're still vulnerable. You can still HAVE your boundaries violated. And this weakness comes from the fact you are separate from others. So the very 'boundaries' that supposedly are meant to establish your security and protection in fact establish a division that renders you attackable. Your boundaries are not strong enough to fend off attack or its effects, because they themselves are based on defense and attack. And there is no such thing as absolutely-impenetrable boundaries.
There is a higher form of this idea of healthy relations. It entails you remembering and realizing that you are ONE with others. You can't really be true to your 'self' (which is a shared self) if you are not also true to the whole. This is where it gets a bit abstract.
Basically, when you are true to who you *really* are, which is the One self that you share with all, your awareness of that truth automatically shows you what is true and what is false. When someone comes along and does something you are already aware of whether it is true or not, and whether it can hurt you or not. If you are in your One self, you are invulnerable. Invulnerability does not need a defense and it does not need artificial boundaries. It is its own defense because no illusions can affect who you really are.
So healthy boundaries are a stepping stone toward self respect and security and being able to assert yourself and to have a sense of protection of your identity. But ultimately they also wall you in and cut you off from others. This leads to the dilemma that you have to 'lower your boundaries' from time to time to let people in and actually have a connection, because if your boundaries are always 'up' you will just shut people out and feel isolated. But then when you expose yourself by lowering your boundaries, it leaves people open to walk all over you. Supposedly the response to this is to 'raise' your boundaries again on an as-needed basis. This is hardly very reliable.
That's why ultimately you want to rise above this whole need for living in a castle, having walls, having a drawbridge that you raise and lower to keep the enemy out when necessary. It's a human-level, ego-level, separate-self-level strategy to use boundaries to maintain a certain level of respect and to honor your sense of ego self. And there's nothing wrong with having that firm sense of who you are and who you are not - as an ego. But you are more than an ego and eventually you have to realize that this dualistic model of protection has a lot of maintenance to it, because boundaries are as much an attack as a defense. They are the ego's substitute for real protection and security, which comes from Oneness.
Being true to who you really are is the ultimate protection because, when you are aware of who you are, you automatically are confident (certain), you automatically see sanely and clearly to be able to discern who is doing what and for what reason, you are able to see that other people are calling out for love rather than attacking, and you know that you cannot be hurt because you are invulnerable. In your innocence lies your safety. Not ego-innocence which is vulnerable to abuse, but Holy Innocence, which is strong and reliable and truly safe.
Also remember, that the only person you need protection from, is yourself. Nothing happens to you against your will, and nothing can affect you unless you choose to affect yourself and agree to it. So how can you have a healthy boundary to separate you from yourself? In that light, if you are fully responsible for your reality and you are honest about it, you will simply stop attacking yourself and thus you are safe forever.
Healthy boundaries are a myth. It's the ego's way of trying to say, being completely distinctly separate from each other is HEALTHY. This is totally the opposite of the truth. It may be healthIER than being an ego with violated boundaries, because the ego feels safer when it is in isolation and separation. But it is NOT health.
No boundaries means death to the ego, on the world level, but no boundaries means absolute safety on the level of who you really are.