I was in Dumathoin back when it was still alive, under the original founder. Dumathoin is confusing as hell, has a long history, is simply just too large, and has many abandoned segments. I've got some general information anyone joining might find handy. I've also gone through and dropped a few signs (IE "Dead End").
This description will be New Dumathoin centric as you will be entering it through Dale, into New Dumathoin. The main cavern the new entrance from Dale tunnels into is from the latter days of Dumathoin. I'm going to arbitrarily label this segment "New" Dumathoin, as that's what it was. New Dumathoin consists of everything immediate to the main city cavern, and the small segment of tunnels off the Dale entrance. The older half of Dumathoin, I'm just going to label "Old" Dumathoin. Old Dumathoin you will find past the tunnel pictured further down, it's off the upper layer of the main city cavern.
New Dumathoin, main City Cavern. Second pic is of decor above waterfall mouth, the beard of which is barely visible in the far left of the first pic. Far right of the first pic you'll notice the main stairwell too.
Entrance to/from Dale from the main cavern. If you go down it, then left at the junction instead of straight to Dale, you'll end up at the second pictured room instead with the water. It goes off to the second main entrance (through a large hall to the right in the pic) that was built which is the third pic. Lots of dead ends in these hallways.
The City Cavern in "New" Dumathoin is easy to navigate as you can actually see where everything is. At the bottom floor with the river chasm was the marketplace and across was the newest and biggest railstation. Down there was the newest forge as well. There's lots of houses along each level, stores, workshops, etc. with scattered storerooms. There should be stairwells recessed in a tunnels up/down the upper two layers of the city.
Tunnel to Old Dumathoin. See the statue thing on the far right? That's up top in the main cavern. Go left here for Old Dumathoin.
Old Dumathoin has many things of interest, and is easy to get lost in. Mithril Forge, Portal Farm, Mushroom Farm, Old Forge, Boat Launch, Old Cavern, and other misc things like the mead hall and original (as in the very first) entrance to Dumathoin. I'll try to post these in the order you'll encounter going out from the city cavern.
This walled cavern is the start.
Here is the first bridge in a chasm/rift with three bridges that span it. Go straight and you'll hit the Mithril Forge which has the Portal Farm off the side of it. You'll pass the second bridge on your left, and the third bridge at the end of the rift is at the Mithril Forge.
Mithril Forge and Portal Farm. The portal farm was never finished.
Third bridge looking at second bridge. Yes they connect to each other via hallways at both ends.
Mushroom farm is at the end of the third bridge at a little jig in the tunnel.
This junction is right after the Mushroom farm, pic is oriented with Mushroom farm and third bridge to your back. The second bridge is through the hallway on the left. Forward is an old railway junction on the otherside of the fence-wall. To the right is the rest of Old Dumathoin after an older cavern.
An older railway entrance, there on the left. Power rails have been stolen.
This here is the old cavern, just past the junction after the bridges with the Mushroom Farm and old rails.
You will come to this junction. This is the area around the original entrance to Dumathoin. Immediately left is the old forge.
Old forge, which behind this is another older rail station.
Rightward of the junction is the Dining Hall and Boat Launch.
Forward through the junction are the stairs to the original entrance to Dumathoin, next to a small hall (left) that goes to the original mines. Older screencap (different texture pack). The actual entrance is currently hidden under dirt.
That's all the major stuff. There's lots of minor things like abandoned houses, old mining tunnels, exploratory tunnels, and minor structures to find too.
That's all for now.