Let me take a crack at it. One of the things that I remember is that their walls can be upgraded past level 6.
Various regions have preferences for recruitment center types. I'm betting they have a higher than all other region type chance of making special forces, and generally have higher values in training and armor/weapons. Something tells me that the rate they fill up with recruits is also special. Either drafts have little effect on morale or reduced. Higher values of draftees, or higher natural replenishment.
Why don't we collect some data and see if your theory is right or not?
I am the lord of Shrine of Seeklander, probably the biggest fortress on dwilight. Here are the RCs I got after almost spending 3k gold.
40 95/50 inf - 15 recruits per day Size 3
50 95/50 cav - 5 recruits per day Size 2
55 80-5/75 SF - 3 recruits per day Size 2
From observing my region, for both inf and cav centers I got low def while the SF center had more balanced stats with low training.
The regions has 10700 population.