ISO: A book about an ancient, ruined wall holding back some destructive force

Question

From what I remember, the protagonist finds a huge, ancient wall and a powerful being (Archangel?) comes to arrest him because the wall is forbidden territory. The archangel ends up realizing that the protagonist was right to come to the wall and ends up defending against and dying to the destructive force while the protagonist escapes to warn the civilization being protected by the wall. I think Shelley's Ozymandias was the epigraph. I read this in the 90's or early 00's. Thanks.