then how would you ever fight a wraith?
Actually, only creatures occupy spaces, as per PHB p283:Just a reminder: When you are moving while phasing, you must end your movement in an unoccupied space. Not in a wall. Not in another being. In an empty space. Unless you are passing through a "thin wall" while phasing (so that the square you enter when moving is an empty square), you can't really shift through a surface when phasing (unless you can shift multiple squares).
Phasing says you specifically must end your movement in an unoccupied space, and walls do not occupy spaces. (They fill them. p284)Occupied Squares
A creature is considered to occupy the square or
squares within its space.
Actually, only creatures occupy spaces, as per PHB p283:
Phasing says you specifically must end your movement in an unoccupied space, and walls do not occupy spaces. (They fill them. p284)
Defeat by counter example:
Some conjurations occupy squares. Said conjurations are not creatures.
PHB#59 said:Unless a power description says otherwise, a conju-
ration cannot be attacked or physically affected, and a
conjuration does not occupy any squares.
Guardian of Faith said:Effect: You conjure a guardian that occupies 1 square within
range.
Why? If you can move through walls (and presumably stay on the same level), why not floors? What makes a floor of stone more special than a wall of stone?I am pretty sure you'd need a fly speed to be able to sink into the ground and rise back out when phasing.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.