In Stage 7, "Some kind of lab", you will start out right next to a punching bag. Attack the bag EXACTLY 87 times (1987 being the year Double Dragon came out) and you will hear a chime indicating you unlocked something. Immediately stop and the game will teleport you to a different area of the stage full of enemies, traps, and platforms. As an added easter egg, every character, including the Lee brothers, all have Yoshihisa Kishimoto's face instead of their normal face. He is the one that developed and made the original Double Dragon. As soon as you complete the area, the secret achievement "A Foot Face?" (5) will unlock.