I can paint a picture in words Sophie... haha.
Castle has various different accomodations: majority single rooms and almost no ensuites. Most single rooms are typical student rooms with the bed, desk, wardrobe layout of a box-room (think typical student room). Most of these are found in Moatside.
One of my friends was sharing in Owengate and her room was huuuuge, plenty of room for two beds, desks, wardrobes, massive windows. Very pretty.
Another one of my friends was sharing in the Keep and her room was pretty massive and was in the castle which is rather lovely.
Basically if you're sharing you get the prettier, bigger rooms. If you're not you get your personal space (yay!) and tend to live with the majority of students (excepting the single rooms in Owengate/Keep which tend to be 3rd years...)
Having said that I've only seen about four rooms in castle so I'm no expert. I may post a piccy of my room up next week after I've moved in.