Normally, beating Level 70 will make the credits roll at the game's end, not let you progress to Level 71. However, to reach Level 71, you can go to Level 5. Ram wall left of the start fives times, then shoot the wall right of the start five times. Gather the key and use the new path to find the warp panel. This will bring you to Level 71. From there, you can progress as usual to Level 75, after which you'll go to Level 61.