Warp from Level 1 to Level 10

When you are at the flashing section of wall at the end of the 1st level, turn 180 degrees and fire at the wall. The wall will then evaporate revealing a clue that will tell you that you are nearing the warp. Move forward and shoot the next wall to reveal another flashing wall. Walk into it to warp to level 10!

Warp from Level 10 to Level 20

When you see the Level 10 exit, turn around 180 degrees and fire at the wall. A new exit will open up that takes you straight to Level 20