My thoughts:
Very few trees within the city, very stone and sculpted environment that is rough and tumble, yet very formal at the same time. Pictures provided below show off the environment very well, but I believe it is much more populated. The top of the hill is segregated from the rest, the "Upper District" where the nobles reside, the closer to the Main Palace the more prestigious they are. Not that Nobles do not segregate with commoners, but commoners rarely have a chance to approach nobles in their home turf.
The lower districts have all of the commerce, the stadiums and archives, and thus noble and standards mix freely, but the meetings of the high command and the events are strictly for the upper class.
Order is kept through a very strict code of laws and with a bunch of brawn behind it, and the enforcers are often very brutal to the people they detain. Secrecy is key in most types of collaborations, but in public any type of meddling is forbidden, a hypocrisy that has led to an extremely professional and experienced environment, where the people of the Upper District are the cream of the crop in terms of skill and secrecy. Obviously the Du Couteau family occupies the top rung of the hierarchy, and its sworn (Katarina, Cassiopeia, Talon) are by far the most skilled assassins and statesmen in all of Noxus. However, with such high standings leads to infighting thanks to ego and differing opinions, and more importantly, their governmental opponents (Swain, Darius, Vladimir) are just as nefarious and enigmatic.
Noxus is ruthless and professional. Any form of weakness or scrappiness is punished and exploited. The environment reflects this solemn and serious environment, gothic in some cases, but for the Du Couteau household and other structures, spartan features and simple features. Nobody but the highest social standings every enter the Du Couteau household, reflective of their residents and how hard it is to comprehend what they are thinking.