Rescue from Fort Kastav

Meet the men near Fort Kastav
Report to Legate Rikke at the Winterhold Imperial Camp, then meet up with the Imperial soldiers near Fort Kastav.

Sneak into the fort
Sneak into the fort undetected (might be an issue since the soldiers tend to run into battle).

Alternately, forget sneaking and take out all opposition.

Free the prisoners
Find the prison cells, and free the Imperials (×5) there.

Take over the fort
Join up with the freed prisoners, that automatically arm themselves from a chest, and clean up the rest of the fort.

Report back to Hadvar
Report to Hadvar about successfully taking over the fort, yields new title of Legate and e.g. a Banded Iron Armor.

Befreiung aus Festung Kastav