The best way is a long straight rod across the top, above the demilune window, and floor length drapes on the rod..preferably able to be closed fully.

Do you need or want to cover the half-round?
If not, you can do floor-length panels on either side of the window-wall and put a complementary roman shade on the window on the right wall.

It would be easier to put a rod above the whole thing, but it looks like you don't have room with that gable.

Alt: Do one large roman shade on the window wall, hung below the half-round and one on the little window.

If you want something on the half-round, you might need to look into a shade built especially for it.

With the deapth of the sill I would install plantaton shutters inside the casing. Either leave the half circle window uncovered or install a built in fan blind that pulls each side side up to the center like wings. Although custom made they are very nice. The windows are very nice but you would want light control and to block the neighboring houses.