The word structure and syntax of these two lines stands out somewhat from the rest of "Leda and the Swan" in that its iambic pentameter is much less strained and the rhythm of the verse is readily apparent (each capitalized syllable is stressed):
the BROken WALL, the BURNing ROOF and TOWer,
and AGaMEMnon DEAD.
Note how there are a total of four nouns between these two lines ("wall", "roof", "tower", "Agamemnon"), and each noun, with one exception is modified in some way by an accompanying adjective. The first two phrases "broken wall" and "burning roof" parallel each other and are even vaguely alliterative, creating a pattern in these lines. However, the third phrase stands alone, signifying a change in syntax yet not in pace -- a natural shift that is not easily noticed. In the second line the word order shifts from adjective-noun to noun-adjective, emphasizing Agamemnon as a casualty of war as opposed to the emphasis on "broken" and "burning" in the previous noun phrases.
This slow and subtle shift presents a reversal that characterizes the entire poem in both content and structure. Structurally, these lines parallel the reversal between the first octave (full of emotional turmoil and side-taking) and the final sestet (in which real questions are asked and the poet steps back to allow a conclusion to form for the reader). They also call attention to the shift in temporal relations, shifting from the octaves present tense to this future tense and finally to the past tense in lines 11-14.
Semantically, the reversal parallels the connection of birth and life with destruction and death, particularly when juxtaposed with the previous line. Most importantly, it is no longer Leda who is the victim but rather Agamemnon, whose death here is a direct consequence of Leda's "engendering" of destruction and of Helen (important also is that only Agamemnon is mentioned, not the many other casualties of the War). That Yeats mentions Agamemnon is very interesting, since Agamemnon was murdered by either his wife Clytemnestra's lover or by Clytemnestra herself along with her lover, a myth which has changed over time, thus giving the wife power over her husband's life -- but, while interesting, this seems to have no connection to Leda's rape. The reversal occurs in an exchange of power across the board from husband to wife, father to mother, man to woman.