Today and last night, I managed to do some more fleshing out of the caves. Adding interesting and surprising details is a primary goal of the game, since it directly supports the sense of exploration as you play to find new and unexpected things.
I'm not going to go listing everything, because, you know, spoilers, but for instance, sometimes, vegetation from outside the cave can now creep into the twilight area of the cave. It has no effect on gameplay, but it implies something about what's going on outside the cave, giving the cave a different feel right from the beginning.
It's kind of difficult to do add detail in a traditional roguelike, where ASCII characters are your palette and the tile granularity is the size of a person. The "graphics" tend to be more about what you can imply to the player than actually trying to be representational.
Quite often, in fact, roguelike "graphics" have the curious effect of not being clear what they mean, which in turn adds more to the game. In your typical first-person-shotoer, when you see a wall, or a plant, or a chair, or whatever, you know exactly what it is and can dismiss it. In a roguelike, all you see is a 'c' character, say, and you have to explicitly go over there and look at it to figure out what it is. "It's a chair." Frustrating, perhaps, but it also has the quality of focusing in on that item. In that same first-person-shooter, you wouldn't give the chair a second thought. That content gets consumed by the player and discarded in a moment...and it probably took the developer a few hours to model and texture and place that chair! In a roguelike, it's there for direct observation and it probably didn't take much time at all to include.
But more importantly, it lets the developer focus the player's attention on important details. By devoting a whole tile to a chair, we can indicate that it has meaning. Games with realistic graphics have to flesh out their worlds with unimportant details to feel real; traditional rogue likes do not suffer that fate. You can make a rectangular room with one yellow ampersand in it, and you may have all the graphics you need for that stage of the game. It's spare, and it's certainly not "eye candy", but it's also intensely focused. There's no confusion in that case what the meaningful thing in the room is!