I have one question about regios, and I've been wondering about this since I've started running Ars: is the terrain in a regio always similar to, in some way, and connected to the mundane realm, or can the terrain in a regio be totally different?
Most of the examples I see of regios are variations of the mundane realms, with this and that added the higher up in the regio levels you are.
E.
As I see it, things are undefined so you could have leeway. I think of most regios as having "fate" link different levels - although there is no direct causal link, events and structures in one level will tend to mimic those on another. For this reason denziens of regios are often interested, and meddle in, the affairs of other levels.
I could certainly see regios that has no connection to the lower levels, however, vast kingdoms of giants for example.
I'm thinking about adding a regio that will actually be "stuck in time", replaying an ancient event or battle. Every level of the regio will have different pivotal moments of this event replaying over and over, and the inhabitants are unaware of this. The player characters can then enter and actually change these events and see how it would turn out, as the higher levels of the regio will also change. Hmm.. 
E.
Sounds like a wonderful idea. Just don't confuse yourself with too many domino knock on effects.
Sounds like a great idea for a regio.
IMS we had the magi visit the recently rediscovered ruins of the covenant of the bleeding rose. This old covenant had previously been a very powerful and wealthy covenant but was destroyed by the fae after numerous violations of the code by faerie hunting.
The whole covenant was in a regio (bound by the covenants curtain walls) and once the characters entered the main gates they repeatedly witnessed scenes from the covenants final battle with Sidhe knights battling hermetic magi, all very impressive stuff which would then fade back to the grey ruins after a few minutes.
The area was haunted too by the ghosts of the coven folk, including one of an archmage who still thought he was fighting the final battle. Not safe to be near.
I like the multiple level regios as a plot device to hint at further things to come and to build suspense.
A ancient graveyard with weathered tombstones, that at midnight, anyone in the graveyard moves to the second level of the regio. In this level the tombstones are freshly made and undamaged and there are a number of mauseleums with open gaping doorways. Entering any of the doorways leads to the next level of the regio, a vast labyrinth, at the center of which is a hidden tomb. Passing the door of the tomb takes one to the final level of the regio, an ice cave with several funery Biers and a great ice throne upon which a dead warlord is interred.
Its a great way of building the story up, especially if, like my group, the characters have the ability to know when they have passed into a new regio level.