General comments:

This level is in fact two separate levels in one. This is the map you'll see if you choose to go the left way.

The first thing you'll meet is a Phantasm. Don't panic you have the Blue Crystal... right? If you don't then you better turn back... quick! This is where talking to the Dead pays off. Of course if you had taken the left ladder on Ground Floor you would have reached this level without finding the Yellow Crystal and most likely the Blue Crystal. Ok so you already knew it existed. But put yourself in the position of someone playing the game for the very first time with no hints. You had a 50/50 chance of reaching this point without the Blue Crystal.

Don't waste your ammo on the Phantasm. Remember what Hans said:

One of those shadowy things enveloped and
killed me. Nothing seemed to hurt it-
bullets, knives, grenades. I ran as far as I
could, wounded, but I finally died here.

If you examine the Blue Crystal the dialog box gives you a clue as to its purpose. It freezes monsters.

Items to find:

Three Walther P4 Magazines (2 are behind a rock)

Points to note:

If you're quick you can trap the Phantasm in the first room.

You'll use more ammunition on this level than you pick up.

If you happen to get killed by the Phantasm you'll find out that Bungie also called them Banshees in the Death Dialog box.

Monsters to kill:

Headless, Zombies and a Phantasm (Banshee)

Decisions to make:

You see a ladder going up. Ascending to Evil Undead Phantasms Must Die! .