I notice that there are several places in the rule book here the designer states that the map building system is based on the 'Enigma' system...any clues as to what this is and if there are any other games that use it?
Yes, these are the two games that I know of associating with the name Enigma system. However, the similarity between the two only goes so far to the name and the fact that map tiles are utilized to reveal the whole map. Medieval is completely more random and chaotic than Blue vs. Gray as players excommunicate each other whoever gets to the Pope. In Blue vs. Gray, history is told in a deeper narrative sense. There is a rigorous OOBs in Blue vs. Gray, going along the historical line, seemingly like tracking the movements and combats of the respective Armies of both sides. With so many map tiles Euroish games out there, it is rather a common game mechanic than anything a beautiful name "Enigma" can suggest, while the term might be very appealing to the masses in the Medieval.
I seem to recall there being a WWII game and something else in design. I have not heard anything about them for years.
I thought the system had more legs than what we have seen but the market apparently does not agree. I think this has something to do with a general reaction against card games following the CCG of a decade past.