The only 'vacuum' level in the game (unfortunately), this one can pose several problems to the novice. The best way to think of this level is as having an outer ring, an inner ring, and the airlock system in the center. Within the inner ring is a shield regen, an oxygen tank, and a pattern buffer, so one can always return here to get back to full power.
To complete the level, you need to work clockwise from the entry point on the level to the antenna. Furthermore, you can divide the level into 4 sublevels, each centered around the 4 large areas.
As the first task, you should circle around the upper right quadrant of the level, looking for a red button while dodging the aliens from the large open area. When you hit the button, look across the large open area, and you will see waayyy on the other side, in the alcove opposite you, lights coming on and a door opening. This can be accessed by returning to the inner ring. Within this newly accessible alcove, there will be a switch that will fully open the necessary doors to get to the next section, namely the lower right quadrant.
Repeat this proceedure, looking for a button and then hitting the switch in the newly opened area, for the two lower quadrants. When you complete the 3rd section, the 4th section should be fully open to you now. The rest of the level is very simple.
If you do happen to fall into the large open areas, you need to pass through the airlock system to return to the inner ring. There is some ammo down there, so a trip to this level will not hurt.
There are no known secrets on this level.