Drop the first mutant and prepare for more action.

Then head straight forward. Enter the purple brick structure through the door at either the left or right corner. Have an officer open the gold key door, make it past the columns, put an end to another mutant and enter the elevator.

Unless of course, you want to find the keys, which is rather difficult in this level.
Navigate the blue maze in order to get to the Silver Key. Make the long tour back and move on by south and west till you get to the north-western corner of the floor. Here the Gold Key is found behind a secret door in a block structure, together with an SS.
There is a short cut back which may involve fighting a Grosse brother, near where you find a chaingun.

This level includes several secret areas with tons of treasure and many 1-ups. But the fighting is tough.