It took her thirty years to build the fortress. See, it was a long period. So it is only logical that breaking through it in just three years was difficult.
To my ear, there is a secondary implication in "He wasn't about to break through ...", namely, that "She was not going to let him break through it ...".
I see something else in the phrase, based on the contrast of taking thirty-three years to build versus three years to break through. 'Wasn't about to break through' could mean nothing more than that the effort would take longer than three years, and that after three years, he was not anywhere near the point of the breakthrough. He wasn't 'about' there yet. Maybe he was halfway, though (on whate