Building lots of fuckhuge fortresses is an integral part of Genosian culture. It's something that they take very seriously as a people, and there are almost no restrictions on the actual act of building castles and forts without permission from feudal lords.
Particularly in places like the Kaselreich, Galli, the Slivers and Iber, the countryside is positively covered in fortresses, and some of them are truly fucking massive. Religiously motivated tithes of labor, an inexhaustable supply of competent masons, and a lack of oversight have made these feudal realms as densely castled as Wales, with fortresses that in some cases could dwarf the Walls of Constantinople.
Of course as time goes by the fortunes of kingdoms fail, so there are also a very large number of decaying, ill-maintained forts, or ones obsoleted by new waterways or newer forts. Many abandoned castles are inhabited by bandits or brigands, or in some cases even goblins.
Obviously in a society that puts this much emphasis on building fortresses, destroying them becomes vital for warfare too. Because of this, siegecraft is a fine art in Vosca, something that no other people, not even the Marju with their foreign engineers or Karthack with its powerful magic, have ever succeeded in matching. Tunneling, counter-tunneling, long distance catapults and trebuchets, counterfire from batteries within the fortress, hidden gates, or even tunnel systems leading some distance away from the fort used for covert sallying and sabotage. Angled wall designs, the use of geometry and kill-zones to advantage the defenders, skilled use of elevation and exterior earthworks, all sorts of stuff that in our own world would not be conceived of until the renaissance all have a home in Genosian siegecraft. And this is before we even get into magic.
Wars that result in the seizing of large fortresses are rare, though the smaller ones change hands fairly frequently. The only time a large fort comes under siege is when the upper lords start coming into conflict over the ownership of whole counties or duchies, instead of squabbling over baronies or border posts.
When this happens, hundreds of thousands of people can die. Very often the two sides will actually agree to fight a pitched battle to decide the contest rather than risk decades of sieges because it will end up killing fewer people in the long run on both sides, but that is usually rare, as the embittering effect of war tends to strip that sort of reasonableness away from people quickly.
Most commonly its uses are logistic. Some very creative weapons can be designed using Sorcery, but it's much easier (and harder to sabotage) to build devices that just make the defending side's life easier. Sanitation is often the big issue, as disease is a constant threat in fortresses. Systems of sorcerous pumps allow water to be drawn up from deep underground, and for sewage to be disposed of effectively without risking some dickbag climbing up the privy, for example.
War machines are less common but still occasionally used. Some forts have truly terrifying weapons powered by sorcery... If you've ever heard of the Winans Steam Gun, imagine one of those large enough to fling a bowling ball hundreds of yards.