I think this song is about getting hurt from a relationship, specifically by letting someone in on your life, your weaknesses, your deep thoughts and hidden feelings. When you get intimate/close with someone, you let them in on a huge part of yourself that you have kept hidden from others (especially when it's your first serious relationship, which seems to be the case in this song since he mentions being younger and less understanding), and when the relationship ends you feel so vulnerable to the outside world because you've opened yourself up to them and now they are gone. So over time he's develop "defenses" from "intruders" who try getting back in, meaning he might be blocking out other people who want him to open up to them, because he is scared of getting hurt again.
I think this song is about getting hurt from a relationship, specifically by letting someone in on your life, your weaknesses, your deep thoughts and hidden feelings. When you get intimate/close with someone, you let them in on a huge part of yourself that you have kept hidden from others (especially when it's your first serious relationship, which seems to be the case in this song since he mentions being younger and less understanding), and when the relationship ends you feel so vulnerable to the outside world because you've opened yourself up to them and now they are gone. So over time he's develop "defenses" from "intruders" who try getting back in, meaning he might be blocking out other people who want him to open up to them, because he is scared of getting hurt again.