The idea is simple!
Took all that you can from the walls and make the hallway, the main living open and bright space.
I think some mixture of the old elements and modern and clean lines could work nice for the place. I mean to preserve all the elements of the wall and ceiling, the old windows. In addition some industrial accents with the high ceiling would be nice too. A library and dining table beside the window hanged over wire ropes from the ceiling. For material of the furniture I suggest natural wood solid or veneer and glossy white, metal details on the seatings and cotton fabrics. Beside the floors in the bedrooms it can be used tiles with concrete decor. I included carpet that imitates faded classical decor. One bathroom can be made near the entrance, you have to brick the window to the stairway (you don't need it anyway). Tiles there could be in smaller size in checkerboard arrangement. I think this is chic and classy. For the bigger bathroom tiles that imitates white marble. That would be clean and give the sense of an old place. For the sanitary I picked a sanitary serie that have classical look but still with clean lines. That could be combined with brass taps and accessories. It is pretty popular right now.
I decided the big room to be the master bedroom. It would be gorgeous! There can be placed big mirror next to the door. The floor definitely must be preserved. In the entrance, next to the door of the bedroom can be placed smaller wardrobe or portmanteau. As I understand there are formed niches that can not be avoid. The two rooms that are formed in the center can be both bedrooms or like in the project a working space and bedroom. My idea is to use the hallway as the living room. In that case all the space is used. The bigger bathroom can be divided into laundry and bath. The kitchen is a bit small but there can be added second row upper cabinets with bigger depth and this still can help for the all look. It would be better if it is without handles.
It is nice apartment even the long shape. I believe it could be made in very functional way both for family or roommates. I hope you find the right people to make them happy! Thank you for the opportunity to work on this interesting project and good luck!