Hi, if you could measure the dimensions with the closet doors open and does the entrance door open inward or outward and where is it located? From what you wrote its hard to say.  You could maybe put a single bed with just a box spring underneath up against the ledge and the ledge could serve as a night stand and create a "faux" head board thats just attached to the wall to create more space.

Thanks @jo mu. The entrance door is where I took the pic from and opens inwards. I'll measure it now

@jo mu so the wall is 220cm and if I open the wardrobe door there's around 177cm. The other wall is 195cm in total and 158cm if we exclude the ledge.

I would stick with a bed frame that is low to avoid making the bed too high

We have a guest room office. I had a custom cover made in a heavy navy twill for the twin trundle bed that reached the floor. In the same fabric, zipper pillow covers for two wedge bolster pillows that side by side are the length of the bed and make a backrest. Then two square accent pillows. For guests, I just remove all the pillows and the bedcover. Replace with a quilt and bed pillows. You can also buy twin daybeds. But I already owned the twin trundle bed.

Just build a platform for the bed that utilizes that shelf as support for one end.