The two secrets in the first level are pretty cool. As the previous
poster mentioned, they include a warp section that lets you jump to
any level in the demo. Here's how you get to them (spoiler):


First Secret (get some good weapons)

Kill the two flyers, go down the hole that opens up. Flip the switch
and grab the ammo. Run right and use the teleporter. Then, go right,
stop before you fall down the pit. Jump across the pit to the little
ledge. There's an invisible teleporter there, but the instructions to
press down to teleport will appear. Press down, you're in the secret
area. Grab the ammo and you now have the grenade launcher and the
rocket launcher. 

Second Secret (you wanna go where?!)

After you drop down from the bonus weapons, to the left you'll find a
save station. Just to the left of that are some ceiling tiles with
cracks. Blow away the ceiling to reveal a jump-booster. Jump up and to
the left. Run along the upper corridor until a tube opening
appears. Go through it and you'll be in a room with four end-level
thingies (those multi-armed numbers that look like robo-surgeon gone
bad). Each one will take you to a different level...


Hey, who's actually maintaining a FAQ around here? Looks like it's too
late to start my own. :^)

Nick
-- 
 +------------------------------------+                                        
 | Nick Vargish, BBN Planet (SURAnet) |  In the words of the immortal          
 |   vargish@sura.net  301/489-8134   |  Socrates, "I drank _what_?"           
 |    http://www.sura.net/~vargish    |                       [Real Genius]