Agatha is guarding the Sublevel, Ulm the Midlevel and Tir Na'og the upper realm... at least untill they attack each other.
Hm, setting monsters there sounds interesting.
Nidhöggr guarding between the lower level and mid level and Mimir, a Giant with massive Astral power between mid and top.
And one idea, I really allready thought before: The middle layer (Uthgard) is surrounded by water - but this is no province. One could make it a province and place Jörmungand into it
Gandalf: Yes, the Land of Milk and Iron - Underworld - Stygian Abyss.
THe big diffrence is, that that map Has every province connecting up/down, while in the tree, you need to go through the bottleneck to chance the layer. This and the fact, that I used Mountains/rivers/a fisure to block pathes makes my map a lot more fractal. In other words: It's just a similar design.