How can I convert fort Hraggstad to Imperial? I already did Joining the Legion quest, but after some time in that place spawned bandits. Only indoor, in prison and main room. Courtyard is empty. Thanks for any help.

If you finished the civil war questline, the forts get back to their previious occupants.

Thanks. I finished Civil war. All forts? What about Greymoor? It occupied by Imperials after battle for Whiterun, and now to. Neugrad near Helgen? I cleared it in begining of my game and bandits changed to Imperials.