Watch out for the officers behind the plants when the action starts.

Make it to the room with the columns to get the Silver Key. Then tour the floor counter clockwise till you get to the north-western room that holds the Gold Key. Continue fighting off guards and officers through the central parts on your way to the elevator. It is hidden behind a secret door near the north-western corner of the floor.

Find a 1-up behind another secret door in the second outdoor area in the north-east.