1) If you can't figure out where or how to fit curtain rods, you can find curtain sconces (? not sure if that's what they're called) and hang on wall at each top corner and run fabric through those. You could even use three, one above the small semi-circle window with the other two on each side of the lower windows and drape your fabric between all three. But because of your radiators, I would use simple and sophistocated shades to block out light.
2) You could use dramatic curtains on curtain rod below small top window and have it all pulled and tied back to the side without the radiator to keep fabric at a safe distance.
3)The two rectangular windows could have a curtain rod running above both windows with panel curtains to drape on the sides and between both windows.
Make sure you post the after pics!