while playing skyrim i have noticed if you have cleared out Fort Neugrad first then let it repopulate with legionnaires then do the start of the Legion Side of the Civil War after you complete the quest Joining the Legion Fort Hraggstad will NOT be occupyed by the Legion i have done this more then once just to see what might have caused it not to work and it seems to be Fort Neugrad being cleared of Bandits allowing the Legion to take over it.
And before anyone asks the Fort just respawns Bandits as i have cleared that fort out many times and by that time of my character i only recently got out of Helgan i useally clear out the mine near Riverwood, bleak falls barrow then Fort Neugrad for loot before advancing the main quest.


I have the save before i got the recent DB DLC on my PC with the game just to see if i could find a way to use console commands to sort out the fort then transver it back onto the Xbox as i wanted to have all Civil War Forts under the side i chose as i have worked out all the other forts but Hraggstad is annoying.