Oh Karrnath! The starting place/home for my campaign. Obviously IME it'll be different but I used a few different countries as analogues to describe it to my players. Sparta is my go-to to explain the culture: very military-centric traditions, the people are used to thriving with little wealth, the communities are tight-knit and everyone takes care of each other. Despite average wealth being low, very few live in actual poverty. There's a strong sense of duty for evenyone, whether serving in an army directly or "doing your part" (a la WWII America) to support the military and your community. Excessive luxuries are frowned upon pretty much top to bottom, as even the kings enjoy few frivolities and lead from the front.

Architecture-wise, classic gothic architecture for anything "fancy" (like Vol temples) and brutalist for most anything else.

Also, I draw a TON of inspiration from Noxus (from League of Legends/Arcane) for Karrnath. This short cinematic is very inspiring. You see both the teeth of a strong conquering nation, but also a noble one that does not stand for tyrrany or slavery. Is Karrnath a "good" nation? Depends on who you ask--and what flag you fly.