Here, the speaker describes a set of emotional defenses as if they were physical walls. They were tall, about three cubits, and they did their job: they kept the hurt out. But the speaker admits something uncomfortable. These walls were "built on betrayal," so the protection came from an old wound, not from peace or trust.
The shift comes with "But now they were crumbling / Faster than lightning." The speed suggests the speaker has little control over what is happening. The final line, set apart from the rest, explains it: "Nothing could break them but his touch." One person, and one gentle act, undoes what so much pain had built.
The mood is tense and vulnerable. There is relief in the idea of being reached, but also fear, because the shield that kept the speaker safe is disappearing. The poem leaves open whether that is a loss or a release, and that uncertainty is much of what makes it linger.