I just did some quick fly-arounds, and did some deducing.
The tall round 'pipe' towers are not smoke stacks. They are support pylons that carry power to whatever was at the top. If you look at the one laying down between the rocks, a bit of the outer structure is loose. Under it, you will find a ton of conduit.
The square towers look to be designed to carry fluids.
There are remains of steel buildings and fluid tanks at the base of the towers. The steel buildings have lots of grid-type windows, like you would see in warehouses or factories, not housing or defense. They are too small for factories, so I am leaning towards warehouses.
There is a very large vent in the north of the map, indicating there is something extremely large underground.
There is also a train station somewhere nearby, either destroyed, underground (the tracks do enter a tunnel), or off the map.
The structure in question is large enough for any ship to easily pass through, and it does look like an extravagant gate.
So, whatever it was was very tall, supplied power and fluids (fuel, likely), and likely supplies to whatever was at the top, and could easily accommodate internal passage of ships. It is also part of a much larger underground structure.
The only conclusion I can draw from this is that we are looking at the remains of a massive skydock, likely for supplying Leviathans(Juggernauts) and smaller ships with fuel, supplies, and lift gas. I would say it is pre-war, since it is both fancy and indefensible.
