you can control its depth. you used zero in your code.
you cannot control layers with actionscript. objects created
in the authoring environment are assigned a depth by flash and you
can control (to some extent) the depths it assigns by placing
objects on different layers in the authoring environment.
if you want to see what depth flash has assigned for a
movieclip created in the authoring environment, use the movieclip
getDepth() method. if you want to see approximately what depth
flash has assigned for a non-movieclip object, sandwich the object
between two movieclips and check the depth of the two