All external marks of abuse are present on this defiant edifice- all the physical features of
And, just when we were happily back in the water, it looks like we're now back on the cliff ("this/ defiant edifice"). An edifice is just a fancy word for a structure or building of some sort. And since we haven't seen any other structures, it's safe to assume we're looking at that beat up cliff again.
Okay, so those "marks of abuse" are pretty easy to spot (they're "All […] present"). And we know that most of them are likely caused by the water driving that "iron wedge" into it. Again, this is all figurative language, so the takeaway point is that the cliff has endured a lot of abuse. Even with its own iron edge, the scars are easy to see.
But just like we saw before, the edifice is "defiant" and withstands the abuse. "Abuse"—that's a funny way to put it, don't you think? We mean, who ever heard of cliff abuse? People tend to be more what we think of as victims of abuse, so the speaker here is making an extended metaphor in which the cliff seems to stand for humanity (and not just guys named "Cliff").
Also, by using the word "edifice" instead of "cliff" here, we get the sense that there's a man-made vibe to this structure, since the word "edifice" usually relates to a building of some sort.
Notice too that the speaker specifically says "external marks," rather than just leaving it at "marks." So there might also be a suggestion that there's more abuse that we can't see with our eyes, internal maybe.
For now, though, we're focused on what can be seen, the "physical features" of… of… yup, it's time for more stanza enjambment.