@ Dave Coyne, I tested it again a bit longer and you are correct. I replaced the keep with a Tower. If you capture the flag now, there will be no new lord spawn and the purple troops will stay hostile.

Same for me , i just captured the flag but after a little while my knight suddenly appears on the castle and it becomes my friend , i cleared all my troops away from flag and out of castle before this happened to see if purple reclaimed it , but they didnt and my knight just appeared

Maybe that is the Problem. try to only capture the fief flag, this should work.

After killing the blue army, i fought against the purple enemy. But after clearing the first fief the purple guy became my friend, maybe because i captured the castle? After clearing the first purple fief, the armies kept spawning but they were friendly, so i couldnt kill them

I tested the mission again and conquered every fief. The purple troops remained as enemies. Which last attack do you mean? The mission has two parts.

After you conquered the first castle (green), you will become the new owner of this castle. Then you need to defend the castle against one counterattack (blue). If managed to defeat the attack you win the first part and the second part will continue. In the first part the purple troops will remain neutral. In the second part you need to capture the two remaining fiefs and need to kill all enemies and defeat all invasions.

I hope this will help you to solve the problem.

The last attack doesnt work, the purple enemy becomes my friend after conquering the second to last castle