A desperate opening.

If without a chaingun turn around and run to the south shooting down the enemies ahead if you. Get the chaingun at the south end and start cleaning out the main hallways. There is a 1-up in each corner, and some ammo to find.

The hallways once cleared you will think that this was the worst part. But it is now that it really starts. You need to enter one of the big block structures to get the keys. Each one of the blocks includes a Silver Key and a Grosse as well as a ghost. The guards may have opened the doors to these structures having the ghosts coming after you. The green section is perhaps the easiest though it includes no 1-up. The blue section is the most dangerous but this one includes several 1-ups. Look for 1-ups in the purple and brown sections also.

Finally prepare for a fight with the SS in the elevator section.