Move on and deal with these two guards. Don't get surprised by the Super SS coming from the adjoining corridor. Explore the rooms you have access to in the south-western corner of the floor.

Open the door to this section of the hallways. Take on the Super Officers coming at you from ahead. Blow up the barrels to finish them. Again don't get surprised by the two Super SS in the room to the south, they may go through the corridor and come at you from behind.

Go through the central room to the south and make it here. Kill the guard and get the brass key. Another guard will start trailing you. Explore the rooms in the south-eastern corner of the floor. Find a 1-up behind the secret door.

You may explore all the inner cells you now have access to and do away with the enemies found in them.

Find another 1-up behind this secret door, but watch that guard.

Continue.