Fortress guards are level 52 soldiers. They are around and inside the Black Knight's Fortress; however, their exact status within the Kinshra hierarchy is unclear. Their weapons include swords, spears, and halberds. Although spears and halberds are generally two-handed weapons, these guards can use halberds in conjunction with bronze sq shields.
When you talk to a fortress guard he might say "Of course the officers have some nice, elite black plate armour, but that's nothing we grunts can ever dream of." indicating they are a low position in the Kinshra army, possibly foot soldiers.
If you are wearing Full Black armour (Or Elite Black), the Fortress Guards will instead say "Sorry, sir, but I'm under strict orders not to chat while on duty.".
Fortress guards seem to be afraid of the Dagon'hai, as wearing the full set will result in any attempts to talk with them in "The guard quakes at you in silent fear."