Creating atmosphere?

[Timitzi]

Hello peoples,
I am creating my custom story and have a great idea, but how can I create a tension building atmosphere?
This is a small demo I did for school:
http://www.youtube.com/watch?v=13D0PJWQudg
But I don't think it creates that good of a atmosphere.
Please give me some advice on how to create the right tension building atmosphere.
-Thanks

Pretty good stuff... You can also add more atmosphere by making custom soundtracks, flashbacks, (good!) voice acting and so on.