You need a curved Curtain Rod, if you're intent on having that kind of ceiling with that type of window.

However, as this particular Entrance Hall was built circa 1691, if you wanted the Classic Castlevania NES look, remastered, you'd need a 17th Century Pillared Entrance Hall set of windows, drapes (slightly different than curtains, drapes are meant to cover the entire wall, not just the window, in order to keep a Castle warm), and a flat ceiling (flat ceilings are a staple of entrance halls, while the ceiling you're using is more suited for other means).
You want something like this 17th century dwelling:
