The `z-index` property is used to create "layers" for your HTML elements. If you are familiar with image editing tools, you may have worked with layers before. This is a similar concept.
Elements with a higher `z-index` value will appear to be layered on top of elements with a lower `z-index` value. This can be combined with the positioning in the previous lesson to create unique effects.
Since the `back-wall` element will need to appear "behind" the other elements you will be creating, give the `back-wall` element a `z-index` of `-1`.
# --hints--
Your `#back-wall` selector should have the `z-index` property set to `-1`.