Llanydern doesn't really have cities (the largest single settlement is ~50,000 people (Aberystwyth the nearest Llanydern has to a capital (the council meets there and the university of Llanydern is organized from there) and is located deep in the mountains in the heart of Llanydern)) the settlement pattern is many small towns (10,000 to 25,000 on average) each of which has one major industry (normally) so the population of Llanydern is very urbanized and quite dense (no jokes) but doesn't live in cities (basically the people of Llanydern like to live in reasonably large groups but dislike being crowded). So if you look at the population density of Llanydern as a whole its a high density population.
Basically no cities just huge numbers of towns, so you won't get stuck in a Stalingrad but there will be lots of little street battles which will slow you down and wear at you.
Facing you are the remnants of the Border guard who are well armed trained and motivated but they are not conventional troops so you will get raids, assassination attempts, sabotage, mines laid on roads, ambushes that kinda thing (basically you come over the border they go to ground and when your main units have passed on they will harass your rear areas (
I'm honestly not trying to make innuendo) they are about 150,000 men and women but part of them are not facing you (not all my northern border goes up against Dysanii territory) and some will have died in the early fighting but there will be between 50,000 and 80,000 of them left (original numbers facing Dysanii territory 100,000).
There is also the somewhat understrength 1st army group (600,000 men and women as the fighting force) which will be holding choke points (fords, bridges, etc) and will also be ambushing your forces when they can (these troops are all grade 1 and grade 2 so they are full time professionals and reservists (the reservists are trained for 2-6 years depending on what service they enter and also whether they have been picked for officer/NCO training, then they are called up to serve 2 months of the year in addition to Sunday training).
Then there is the air force which facing you has about 200,000 men and women (grades 1, 2 and 3 (3 is limited to anti aircraft units on the border at the moment)) of which about 50,000 are actually involved with aircraft mainly helicopters, recon, fighter and interdiction (fighter and interdiction aircraft are variants of the panavia tornado as is close in recon) as well as about 25,000 helicopter mobile troops, the rest are grade 2 or 3 anti air craft units tied in to defense of towns and military formations.
There are militia and security forces who are in defensive positions in and around the towns these are well equipped with man portable weapons as well as some anti tank weapons, they have some older model armoured units (think like a town will have at max a section of tanks or light tanks somewhat old fashioned (think about the same level as T-70 soviet tanks) but well maintained) what transport they have is mainly soft skinned or makeshift (there have been reports of civilian vehicles being armed/armoured. They have some limited artillery and have good protection against air attack (mainly tube weapons) and in the first 250km or so they probably number somewhere around 5 million maybe more.
Also any towns along the coast might well have naval troops there (if you live along the coast you go to the navy for your military service)
The 2nd and 3rd army groups are massing and there has been a general call up of all military personnel, the navy and air force are pretty much mobilized but as this has so far been a land war are of limited utility (my fleet when I check what state its in (i.e. is it 10/11 days since I did the news post and if not how much of it will be ready ) will be getting involved with yours.
A logical goal would be the seizure of the oil fields on western seaboard roughly on a line with the mountains as this would slow if not stop my counter attack (or at least limit it).
