Having experienced a creative fizzle with my first Age attempt, Ahlnay, I'm dropping that one for a while and moving on to something for which the ideas flow more easily. Details here are subject to change...
Gnomon is meant to have been written by a human, hence the English name. As the title vaguely suggests, it's primarily a very large rock spire in the middle of a hot, dry, sunny area. The spire will eventually cast dynamic shadows based on a day-night cycle, although I haven't worked out what puzzle significance this might have.
The gnomon (which is what I'm calling the spire now - potentially confusing) is about as large as a medium-sized skyscraper, and is filled with large empty pockets, geologically formed along with the spire (I was toying with the idea of it having been an enormous insect mound, like termites build on Earth, but that would probably mean tiny, non-navigable caves, which doesn't fit the concept). Because Gnomon is meant to be fairly old, the Age's writer (me?) has filled the caves and exterior with all sorts of accommodations over the years: carved stairs, a garden with greenhouse windows, an external catwalk and staircase, an elevator, a bedroom, etc. Some of the caves were cut larger in order to make room for more stuff. The whole aesthetic is meant to resemble a kind of hybrid of the Ages of Riven, Tomahna, and Er'cana, so there's lots of rock, wood, and rusty metal.
The Age is also supposed to work as a giant combination lock to protect a secret in the sub-basement of the spire. The very top of the tower can rotate (yeah, I know, we've seen that before) and in doing so it essentially acts as the dial to a padlock, working towards opening a colossal hatch on the very bottom floor. The writer wanted to protect a linking book that was very valuable to him...
I've been working on this off and on for a couple weeks now, so I can show Blender screenshots if anyone wants to see. It's not ready to export, so nothing in-game is available yet.