I meant tiles before enemies.

and tiles (if you're going to have any) before propulsion. Placing gauss turrets, rocket turrets, laser drones, or chainguns can affect the delay between fires for those enemies, so placing enemies after you've started can ruin the DDA. After you've placed those enemies, just let them fire, and make sure you aren't running away from the action. In addition to this, if you use chaingun drones, remember to begin recording a new demo (press 1) before testing it, because chainguns behave differently depending on the starting frame.
Also, avoid using trapdoors to block enemies. It's fine if you use them for propulsion (I use trapdoors more than any other form of propulsion), though.

for a first. Sure it's not very exciting, or long (compared to many others) but you obviously have the ability to make a decent one. I'd suggest starting with the propulsion and then adding the enemies. After you've got the layout and prupulsion mainly done then you can do trial and error enemies easier. there will be place you will dodge all the attacks of for example rockets and still have really close calls. Also, try to use less trap doors... and look/rate my maps >.>