Thunderhead Keep Glitch : GW Submissions

Play through the mission how you normally do up till the fort part with Dagnar.

Kill every group of enemies till only Dagnar is left. ((Do NOT agro.))

Walk down the stairs and right through either doors at the East and West side of the fort. ((they appear closed))

Run around and find all the mursaats / jades and kill them.

Go back and kill Dagnar to trigger cutscene.

Now you'll have a more easy time keeping the fort.