Go through...

...this...

...or this locked (silver) door.

Enter the eastern part of the main hallway. Try to hit Super Officers around corners with your plasma gun. Gain a missile launcher and missiles. Clean out the hallway. Be prepared for enemies coming at you from adjoining rooms. Collect goods.

Enter this room for more bullets, missiles and health. Beware the Super SS behind the mirror.

Clean out the kitchen area, the dining room and other rooms in the north eastern part of the outer section for even more health, bullets and missiles.

Break the mirror in this bathroom, open the secret door behind it. Get a cross.

Make it to this room for the bronze key.

Continue.