A good explanation feral. Well, in GE we can trick the z axis by using zdepth and positive y axis.
For example make 2 actors.place them on positive x y coordinate (higher than (0,0) coordinate).make 1 of them can move up and down,using keydown or something.then,put this code on DrawActor event on both actor:
ChangeZDepth("Event Actor",y*0.001);
Now you can see that,the lower position(the higher y) will cover the higher position(the lower y). This method usualy used on RPG,front view race,first person shooter,etc
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Oh yah,btw what i mean with "how do you make the z axis" is how do feral draw the diagonal line which connect every side of the front box with the back box...