These "levels" are not levels per se, this present order is just a GateWay homepage decision.
There are entries to other points of time in T.O.W., but I have decided to only describe these three levels. If I suddenly have a Victorian Steampunk rush, I may do a page on that point too.
None of the levels is "higher" than the other, the only order to it is that most people would agree, that the Medieval comes before the Now, which comes before the Future :)
There is also a rudimentary Fourth level, which takes place around The Creation of Man. Here, instead of the humans and simians, the evolutionary tree splits itself up in humans, simians, and elves, trolls and the like. This is a crucial point for the rest of T.O.W., because it is here, the magic, which splitted this universe up in the world I inhabit with you and T.O.W. But there are no characters, so I may write this level in the Library instead. But then again, I may create a page for it too :o)
There are galleries for each layer in the Elfsewer, more info on that in the [ElfSewer F.A.Q.]