All I get is the initial screen with 32 invaders (4 rows of 8) and the missle launcher (which in itself looks pretty good). However, when started, the launcher jumps to the far left of the screen, and moments later the invaders all disappear. Also, an annoying load 'buzz' sound (sort of screechy) is emitted.

The maze game was not the important thing in this scenario, so I did not put too much effort into it. The purpose of this scenario is to make the mapping out of levels easier by giving the users more methods to use to 'prepare' the world and provide code snippets to incorporate in their world class code.

We are wanting the keys to be horizontally centered in the world. The center of the world is at half the width of the world. Half the keyboard is to be on the left of world center and the other half on the right. The whole keyboard width is the number of keys times the width of the image of one key; half of that is the number of keys times the width of the image of one key divided by two. Subtracting half the width of the keyboard from the center of the world gives the location of the left edge of the first key of the keyboard. Adding half the width of one key to that will give the x-location at which to place the first key. The remaining keys can then be placed by adding the width of one key to the x-location of the last key placed.