At the request of 
yhlee in comments to the Readercon structure panel report, some quick thoughts about the structure of the Sarantine Mosaic (Sailing to Sarantium and Lord of Emperors).
This discusses the ending of the duology and contains enormous, book-destroying SPOILERS.
The overall structure is an outward spiral. The characters talk about life as circles, referring to the sun-worship religion, but they're wrong (she says, modestly), because things don't come back to the same place, they find echoes and parallels to the past as they move on. The very ending is clearest about this, though there are lots and lots of examples throughout.
Crispin is in his hometown, working on a mosaic at the sanctuary, and called down off a platform by Alixana. There are three places in the spiral that this lines up with:
- The very beginning, when Crispin was working on a mosaic at the sanctuary;
- The end of the first book, when Alixana draws him down off the platform and back into the world;
- And the middle of the second book, when Alixana brings him down to show him dolphins on the day of the assassination.
Echoes not repetitions; Crispin has gorgeous tesserae (Gisel's note always makes me sniffle) and is signing his work; Crispin is not just in the world but finally took active responsibility for an action, giving Styliane the knife; and Alixana's coming to Crispin was made possible by that day, but with much journeying between. And no invasion is on the horizon.
The outward spiral is thematically suited to the two problems presented at the start of the work. The first is personal, how to go on living when your life has been destroyed; the second is political, the reconquest on Italy Batiara. What's past is acknowledged, referenced, and learned from as life moves forward.
That's the plot structure, its scaffolding. I also mentally overlay a triangle on that spiral, because it's the shifting balance of Alixana-Styliane-Gisel that drives the plot, but that's a little different; they're a triangle traveling along the spiral, if you will.
(A triangle with a bit dangling from each corner, the shadow-figure for each woman: Shirin for Alixana, Thenais for Styliane, and Kasia for Gisel. And with a set of balance scales hovering over, as it's the main (though not the only) example of the central theme of the balance between emotion and intellect. Look, it's a very complicated mental image, okay?)
Other stuff about these books: Spear-points in the Sarantine Mosaic (also huge spoilers, of course).