Take the cross in this room. Then battle the enemies in the rooms to the left and the right. Find health in both of those rooms.

Find also many missiles in this room. Open the book shelf and fire at the armour to the north past the table and the lamp with your revolver.

Run back here. Wait for the armour to come through the locked (brass) door. Defeat it, and the two enemies within. Make sure they fall in the doorway making the door stay open. Enter past the death chalice. More enemies from the south have been alarmed, take them out as they come.

Shoot the guard in this room and reinforce. Find two crosses. Take one at a time...

...go up to floor 2 again here (green arrow)...

...and get these two death chalices. Enter the room beyond them, kill the sub mutant and the guard and get the silver key, and more health.

Continue.