@MistLynx I cannot remember when it was established, but Mauricia and Haurelia are separated by a long mountain range and a very large river. The only opening between the two countries is along the border between the Cornelius and the Selvy domains. The reason this is important here is because Antrim was originally part of the Haurelia kingdom, meaning it was on the eastern side of the mountain range. It's supposed to be bad to access from the west, since there's a natural border between the two places.
It is a small ass-pull by the author to declare that only one notable road exists between Antrim and the Falkirk territory, but the situation has been established this way from very early on. In that sense, whether or not there's a fort there doesn't actually make much difference. The possibility explained here is that you can just spend some effort in collapsing the pass through artificial landslides. It won't stop the enemy army, but it will notably delay them, which also holds at least as much strategic value as a fort would. Antrim is expected to be the first target if a new war were to break out and due to it's position it'll be impossible to send sufficient reinforcements to actually stop the enemy army in any realistic amount of time, which also means that they wouldn't be able to hold a fort for long enough to send backup. This, in turn, would result in the fort being a very costly endeavour that'd grant the enemy a somewhat-free stronghold to defend against a counter-push.
@Scrwd The difference to regular trench warfare is that Balud's side has built their fortifications and trenches in a way that allows quick escape, but offers no advantage to the advancing side in case they manage to capture said trenches. Large sections of the trenches and most of the fortifications are open towards the Antrim side [no walls and simple ramps, respectively], which means they fulfill their purpose when used by the defenders, but won't offer assistance to the invaders once captured.
