Floors in plural would usually refer to different levels in a house. 1st floor, 2nd floor etc. I suppose that a definite article was used because rooms tend to have just one floor. Probably the same would have occured with "ceiling".

The room has a window - this means that there is a single window in the room.
The room has windows - obviously more than one.
The room has windows facing towards the beach - more than one, and more than one facing towards the beach, but possibly others facing elsewhere.
The window in the room faces towards the beach - in this case you state that there is just one window, and it faces towards the beach.
A window in the room faces towards the beach - here we can be sure that there are more windows, but none of the others face towards the beach.

Not possible: An only window in the room faces towards the beach. You can't use "an" and "only" here at the same time. It's a contradiction: "An" implies more than one, and "only" puts the number at exactly one.
Not possible: The room has the window. You would expect some additional property to define this specific window as a reason for the article.