Before I was born, mom once made a mini guide for Alpha Mission for the NES. In it, she mentioned that there was a playable Area 13 which you will fight the bosses in a boss rush. Nowhere else on the net has there been any mention of it. Not even on YouTube till now. At around 23:23 you will see a playable Area 13. Defeating the Area 12 boss the conventional way will display Area 13 then jump to Area 1 immediately.

https://youtu.be/oz0VrI0ya0Q?t=1403
Gameplay by Chigura "Kage Negra/Tiggy Smalls" Castillo

You can view the reference guide here:
http://www.ign.com/faqs/2006/alpha-mission-general-faq-716432

Anyone else knew about this level?