Just throwing ideas out:
I wonder, maybe the code is hidden somewhere in the order you train with assorted MK Characters.
Say, you train all the mandatory characters first, then after Konquest, train with all the optional characters, like Kobra, Kabal & Noob Smoke.
Or
Maybe there is some way to get back into the city, because there was an entrance way, that you couldn't get back to until you become a monk...