There’s no functioning purpose for the portals and the only purpose of the NPCs guarding it is to make it slightly more difficult for a stealth enemy to freely move around inside the walls.
Does anyone knows any lore-wise explanation for those structures (daedric portals? shields? Other?) They are ubiquitous in every Cyrodiil's keeps regardless faction and seems to be maintained by two NPC warriors or mages.
It not seem useful in battles or any other situation, but I was always curious why they exist since I saw them first time. Thanks for any comment.