Ok, so I'm totally new to the lore, but I'm trying my best to learn.
When I hear about frozen wasteland and underground cities, I can't help but think of
Krieg Corps from Warhammer 40k. Living on the planet so destroyed they sometimes spend whole lives
without taking their gasmasks. Their hive-cities lay underground and they are the masters of defence.
(Of course they are also based on WWI germany, so your photos only made me even more convinced of similiarities)
You'd think with nothing but technology and power, they really should be masters of defence. THey have to protect the
technology they salvaged, or else they would win fairly quickly, being outnumbered by fractions with more resources.
I can see their borders as nearly unpassable, with high-tech turrets and underground(underice?) tunnels connecting various
defence structures. Of course they dig a lot, they have machines and manpower for it. Also, digging in ice is more resource-friendly
than building ontop of it.
I also had an idea for farms and such. They could be subturrean, placed in like, big caves under glaciers.
Think of this:
http://elephantwords.co.uk/wp-content/uploads/2013/04/icecave.jpg They would have that minimum of sunlight that's needed for plants to grow. Of course they would be mostly eating plants that don't
need much light, but the warmth is not a problem. Big brass pipes running around those plantations would heat the air around them enough for plants to grow. The ice above wouldn't melt, becouse outside is freeze-to-death below zero degrees, and the ceiling would be far enough not to get a direct heat. I also tought of glass ceilings for plantations, but I figured glass would be far too expensive i the land with no sandy beaches.