There should be a quest for some strongholds so that they let you enter, also i was able to get to all strongholds after killing bandits for a farm near markath, there was an Orc who was there when i told the farm, then i started to say:
''i will inform the Orc stronholds (or something like this) that you can enter, you are a good man (or something like this).
Just do something for an orc