You see, one of the first races of Units to come to the Map of Ulgress were Demons. Demons are over-the-top Role-Players. 

Role-Player
n. One who entertains oneself by pretending to be somebody else

Silly, stupid Demons got it in their heads that the Map of Ulgress is a world of medieval fantasy, so they ran around blocking the paths of the Map with elemental Seals. If we can't find the matching elemental Crest for each Seal, we'll be stuck on this side of the Map!