They have mentioned dwarven lands, etc. being explorable and I would assume that there are npc settlements there that we can attack. Aside from that..
I would say that this could be implemented in two ways.:
1. Roving NPC warbands. They could have roving NPC warbands that function kindof like rebel armies in Rome: total war. They generally do not attack your strongholds (because they aren't often strong enough). However, their presence near your stronghold weakens you. It reduces your production. Therefore, you have to leave your stronghold to go kill them. These wouldn't be a constant threat, more of an occasional nuisance to throw some variety into the game.
2. Very rare npc attacks. NPC armies that raid your stronghold very rarely, more of an event than an occasional happening. Perhaps a powerful force that appears and attacks several strongholds before either being destroyed or leaving for another area.
"He uttered the word of curse, crush him with our Orcish iron!"