Join the men attacking Fort Kastav
The attacking force will gather to the Southwest of Fort Kastav.
The forces will gather to the Southwest, at the base of the hill the fort is on.
Take over Fort Kastav by defeating the enemy
Time to climb the hill and assault the fort itself. Expect to encounter a few enemies on the way up to the main entrance of the fort. Break through any barricades in front of you with a few swings. Once again, all the fighting will be on the outside of the Fort itself. You will not need to enter of the buildings to clear the quest.
Work carefully to attend to all of your opponents. Have your Stormcloaks aid you as much as possible. You will want to stay with them if you are melee oriented. For ranged fighters, just watch where they are.