Can't you simply use ramps and have the floor levels vary just slightly (like in Doom)? Can't imagine the AI has problems with going up a ramp.
Anyway, nice start. The floor tiles are maybe too thick. I look forward to more shots once there are some details. Remember to put a focal point in each main area (crane, bridge, generator, helicopter, etc) so players can easily tell them apart. Basically each area should have something unique to it.
Also perhaps put some dedicated choke points in for gameplay.
Looks good, textures seem to work fine. Remember functional lighting.
http://www.worldofleveldesign.com/categ ... enssen.phpThere is a package with free mapmodels here - perhaps useful for decoration if your mod can use mapmodels (misc_model or similar).
http://www.md2.sitters-electronics.nl/