Complete the "Breaking the Ice" level by defeating the final Boss. To reach the bonus level, Go to the main room in the Batcave and fly up to the Bat-logo elevator located to the left of the Batcomputer. Once you are in in the Trophy Room, fly all the way right to access a studio set that resembles to the Wayne Manor study featured in 1966 Batman television show. Swap to a character with a grapple gun. Pull the grapple point found to the left of the bookcase to reveal a secret passage. Jump on the poles and slide down to start the bonus level.