I think the answer here depends on what you want the curtains for. If they're for light and privacy, I might consider what the previous owners already started to do and get some custom blinds or honeycomb shades (probably with remotes for those guys at the top). If they're strictly decorative, I'd hang very long drapes from rods placed above or level with the top arch, but just on the sides.

Or, do both together!

You can go level with the top arch and short rods and finials. But I would consider hanging drapes on either side of windows, but install as high up as possible

There is always the option of custom arched drapery hardware.

Same window design. Our problem is that this beautiful window faces the west-southwest allowing significant light and heat enter our Great Room. Consequently, this room is not usable for several hours in the afternoon. We are thinking of blackout drapes on three rods. Adding rods to match coloring of the wall. Pull back when wanting the view or light.

@John: Start a new Dilemma thread. This is a 7 year old post. You will get more help if you show photos of your house and tell everyone what your goals are.

Any help would be appreciated. We have a slanted ceiling.