After reading about Acheron I still have some doubts about it...First layer Avalas consists of many iron cubes that collide into each other and this is also a layer with most inhabitants on it. Two major realms are Clangor and Nishrek and it is said that they are heavily tunneled and carved. What bugs me is this...are goblin and orc cities/fortresses build among those tunnels, that is underground or they are built above ground? Same goes for the Mercykiller fortress Vorkheon (sp?)
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My reading is: mostly underground. However, armies have to march across the surface to get to another underground fortress to assault it, and lots of battles happen on the surface.
And the cubes, in defiance of probability (which only an addle-coved barmy would think is relevant on the outer planes) tend to hit face-to-face when the collide, squishing all and sundry.
I ran one game on Acheron. The characters were all orcs, and they were dispatched to assault a small goblin garrison -- which was around a recently-discovered portal to Sigil. The orcs, naturally, would much rather control that themselves. The orcs went to fight, and found out that a small number of Mercykiller humans (et al.) were working with the goblins. They triumphed, but decided to just go to Sigil and seek their own fortune rather than bring reports back to their leader. It was quite a fun one-shot to run. (This one I ran using Fudge.)
-Rob