Metal Dome

As the party passes through the mercurial dome entryway created by their riftforged companion, the door closes behind them, leaving them in a strange land. The air is cooler and drier than the tropical weather they were just in, the sky overcast instead of sunny, the sun setting when it was noon. Behind them, they can still see the tropical climate they just left, blocked by a shimmer of magic, flowing like water.
The land is green and grassy, with shrubs and small trees scattered in the immediately surrounding area, a forest of deciduous and pine trees grows to their left. To their right, a large canyon with a disused path running along the northern side the canyon edge towards a switchback path running down the cliff wall alongside a massive waterfall. Down the side of the cliff, in front of the waterfall, is a building, metal roof shimmering in the setting sun.

Approach

You stand on what is presumably the north side of a canyon that travels east to west. The setting sun lights up the whole canyon, the stone walls a reddish-brown. With the curve of the canyon, you can see in the distance, a building shimmering in the red-orange setting sun built into the side of the cliff, in front of a waterfall, attached to the cliffside on either side of it. There is a clear path that goes along the cliff side and down the wall. 
By the time that you near the switchback path down, the sun has set. The night is dark. The shine of the moon and stars can't pierce the overcast sky on this cloudy night, leaving the party in near complete darkness in an unknown land. You hear familiar sounds from the wilderness, the sounds of insects buzzing and chirping, small toads croaking, and an animal howl as well as the constant drone of the waterfall. 

Switchback & Approach

A 10 foot wide switchback path with established ruts from the carts that once passed through to goes down the side of the cliff until it levels off with the building where it goes straight along the sheer cliff to the building. 
A wagon is parked in the entrance area of the building, its wheel broken and tipped forward. The canvas cover of the wagon is worn through, tattered and blowing in the cool breeze (relative to what the party has been used to), showing the skeleton of the wagon that once held the canvas in place. 

Flyby Information

If they try to scope out the condition of the building - they find that the roof is mostly intact, although rusty. There are a few holes where vines have broken through the rusted roof, making their way inside the building. The one notable observation a flying creature can make about the building is that there is a walkway that goes in front of the waterfall area, where the shelf continues.

Exterior

The building appears to be made entirely of metal - note this is not something you have seen before. Metal is still a precious resource that is used to make nails, horseshoes, weapons, and armor - making a building entirely out of metal would be a waste in most people's eyes - why not make it out of something more practical? The roof, also made of metal has rust that is starting to build up on the edges. A quick glance shows that the building appears to be falling into disrepair.
DC 20 History: None of the races you are familiar with used construction such as this. The building appears to be more for function than aesthetics.
Plants surrounding the building have clearly not been tended to, grass and shrubs growing high along the walls. Some vines are poking their way through the rusted portions of the roof into the building, small trees growing right next to it around the wagon that was left behind.
DC 15 Nature: The building probably hasn't been maintained for around 75 years
There is a shelf of some sort, about knee height (for most of the party), with tiles laid across it and pieces of metal stacked upon those. Presumably the tiles at least were once in a neat order, but now vines and grass have taken over, some of the tiles have been torn off, plants grow atop some of the tiles, while others have plants growing through cracks that formed over time. The shelf goes into the building at, what appears to be a small metal door only about 2 square feet in size, that is currently closed.
There is also a metal door to the right-hand side of the building entryway. The top corners and handle are rusted and two foot high grass grows in front of it. 

Office

This room appears to have been some sort of office. A desk and chair are placed directly across from the entryway, moldy, damp, and disintegrating papers are strewn across the floor and on the desk, what appears to be a small arsenal of weapons and shields are on tables to the right as you enter, and vines that have broken through various cracks in the metal walls and ceiling and are flowing over the floor. A slateboard with drawings of two mechanical humanoids with various weapons and armor, numbers, and arcane runes drawn upon it.
There is another door immediately to your left as well as two others - one directly next to the desk, exiting out straight across the room and another that is on the 'north' wall. 

Storage

The doorway straight across the way appears to open what is a storage room. Wooden crates braced with metal are stacked at the end of the narrow walkway area. The crates are filled with a strange type of tubing.
Researchers of Riftforged Information

Those who have studied the construction of the few riftforged remains would know that the tubing found in the boxes is similar to the tubing that are found inside riftforged.

A strange crank doorway of sorts is located to the immediate left of the doorway, leading directly outside to the cliff and waterfall.

Hallway

Through the door you find yourself in a hallway that makes a right hand turn, preventing you from seeing further. You can clearly hear the sounds of the waterfall nearby.
When you go around the corner you find two doors - one to the left and one to the right - and some stairs that go up and to the left - preventing you from seeing further.

Observation Area

Through the door to the right in the hallway there is another narrow hallway with a door at the end. You find what appears to be some sort of observation booth. A chair sits in front of a working area with mechanisms you have never seen before. More vines have broken through the glass observatory bubble and are growing along the floor. 
If anyone tries to press any of the buttons, nothing happens. 
Upon entering the room - four of the vines start to move Battle with 4 vineblights

Forge

The door to the left is a much heavier door that takes a bit more strength to open.
On the other side of the door you find what appears to have been a forge once. The smell of soot and burning permeates the air.
This appears to be where the small tile shelf outside lead to - metal ingots are on that tile shelf as if they were meant to pass over the forge and heat / melt and prepare for smithing. The tile belt continues through another small closed metal door into another room.
DC 15 Nature Check: The forge used magma to melt the ingots, now there is only a hole that goes down to where the once active magma flowed. This makes no sense given the proximity of the forge to the cliff and the waterfall. 

Waterfall

You continue through the hallway - where the sound of the waterfall gets louder and louder, until you find yourself on a metal walkway with rails that goes directly in front of the waterfall. Vines are growing all across the path.
There is a metal mill that is using the force of the waterfall to turn - but it does so in jerks that shake the walkway. Time and lack of maintenance has not been kind to this metal mill as some of the spokes of the water wheel have broken off causing it to not function properly.
The tile belt continues alongside the walkway - closer to the waterfall - where instead of metal ingots, there are now pieces of what appears to be riftforged or some other similar mechanical humanoid. Based on the path of the tile belt, it appears that the waterfall was used to help cool the metal pieces.
The tile belt appears to continue on to another area - once again blocked by a small closed metal door.
DC 15 Perception - off the edge of the walkway - you notice that there are gems embedded into the shaft of the water wheel DC 20 Athletics / Theive's Tools - to be able to break one of the gems off without some sort of tool / dagger - DC 15 to break it off with a dagger
One thing you all notice is that on the outside - alongside the far edge of the other side of the waterfall - you see two riftforged standing.
DC 15 Perception - they appear to be inactive, vines have wrapped around their legs and up their torso - as if binding the riftforged in their place. 
If any of them mange to see into the other side of the building...

Riftforged

In this room stand 23 riftforged - all stationary, not moving, with no signs of life. One of the riftforged is set off to the side - as if it were moving, but never got to where it was going.
Vines grow all over the floor, some managing to wrap themselves around the riftforged, binding it to the vertical bed that they stand in. To the far right side of the room is a huge crystal embedded into some sort of desk that slightly resembles what you saw before - with switches and buttons that you have never seen before.
The 'northmost' and 'eastern' walls are made of glass, allowing you to see another intact riftforge laying on the tile belt and one final riftforged laying down on a platform in a side room.
The side room has a riftforged laying down on a platform - gems are embedded into devices in each of the corners of the room, pointing directly at the inactive riftforged on the ground.
You also notice that a much larger tile shelf comes out at the top of this room into another 10 wide path of sorts.
DC 15 Perception - You notice that the tile shelf of riftforged parts does not appear to go into this room and there isn't a door to the left that would go where it went. It appears the larger tile shelf that the riftforged to the north is laying on might come from another room where the smaller shelf went to.

The metal dome appears to be that of molten metal, like mercury, ever flowing, but impermeable. What could be on the other side?

Type

Shelter