Heroes 6 - Discussion

When I first saw the object for the underground terrain in H5 – a sort of hole, a basement, I would say – the idea of a third level of the dungeon arose. And, actually, why not? The simplest interpretation is that the lowest underground level is inhabited by demons, higher up live other lovers of confined spaces, like dwarves and the Tower, and on the surface, all the others. But the heavens don't seem very appealing. The underwater level is okay, but it seems that only marine creatures will be able to move there? Or will they be underwater caves? If the latter, how do they differ from ordinary caves? It's unclear.