Hit the character limit on that post, I used this tutorial:
http://lodev.org/cgtutor/raycasting.html
It was written in C++, and I ported it to GS2.
There are two attachments named gunderak_sky.png and gunderak_floor.png, they are just 1x1 images, blue and gray respectively.
Best of luck clicking them.
My desire is to have people learn from this, and possibly build some minigames in GS2! As I don't have the time anymore to do this.