Alternative route to the fifth door

After the fourth door (the first one on the third floor), the normal route would be to go left through the opening. However, you can instead go past that opening, until you reach the edge of the shaft that runs up through the level. You can then clamber over two ridges to approach the next door from a novel angle. It'll be nice for doing evil deeds from afar with the rocket launcher. But watch out, you'll get a nasty surprise along the way!