Go down to floor 0, the basement (green arrow).

Find yourself here (red arrow). Find ammo around the corner to the west but watch the bomb. Kill the guard around the corner to the east by the wooden barrels.

Break this mirror and open the two secret doors. Beware the sub mutant. Get the gold key.

Find health in the washing room. Push the rightmost shelf and reveal a 1-up.

Enter the vine cellar here. Deal with the guard hiding in the niche to the left (to the right on the map). Switch to the axe and handle the sub mutant and the ghost. The ghost may already have come at you or hid in the room in the south western corner.

Locate the secret door in the niche. Kill the Super Officers safely by blowing up the barrels. Watch the poison puddles. Open the secret door for the cross, and the next one for another 1-up.

Continue.