Here are 7 Duke maps. They all bear the original file name (E1L1.map, E1L2.map...etc.), so, if you've extracted Duke maps for build, then rename them. I use an "a" after name, such as: E1L1a. Then unzip files to Duke3d dir. When you want to get rid of them, del and just remove the "a".
 In my style of mapping, one can usually take clues from the view screens, as to where you should go. If there is a switch around a screen. make sure to look at screen after you flip switch, as you may see a door open, a ramp move, etc. A lot of times, there's only one way in, not too many false leads. Areas open up as you go through.
 Remember too, that E1L1 was my very first attempt, and maps get better as you go. I didn't even properly use the Cycle sprite until map 5 or so. Would appreciate any feed back. I also have a map called: Sample, that I use for reference. I constantly update it, but it has most all of the textures displayed and sprites. If you're interested, let me know.
Thanks for your interest.
Steve