Just a quick example to show how you can use a horizontal skew to create the illusion of a floor with depth.
The trick lies in creating a trapezium graphic which depicts how part of the floor looks when viewed in the centre of the screen. Then the floor skews horizontally in such a way that when placed next to another trapezium their sides will match up in a 3d mannor.
However, at the end of the day...its a hell of a lot easier to just use a 3d box, but I thought this example might be interesting anyways.
PS: credit goes to Vrav and Holymonkey for the skew idea in the 'streetfighter floors' thread.