Before I answer you I am going to tell you that I tend to be really good at interpreting dreams. A lot of the people I am close to always comes to me when they have a dream that is bothering them. Okay onto your dream.
In my opinion since you are sitting on a window sill in a tall building looking down its your subconscious letting you know something important. Think to yourself? Were you afraid of the height or were you calm? If you were afraid then you might feel somewhat out of control in your waking life. Something in your waking life is making you feel as though you are going to "fall". If you were in this situation is your waking life ask yourself how you might feel? You might be afraid that you will fall and loose everything. The same goes with the metaphor in this dream. Or you may have set a goal that you might feel as though it is to high for you to achieve. But if you were not afraid then you may have reached a goal in your waking life that has taken you a long time to achieve. But keep in mind that the fear of not being in control and the feeling that you might be on the verge of giving up is something that all of us feel, but you cannot let yourself do this. Just keep going! No matter what! Keep working towards your dreams! <3 <3
Hope this helped!