Stronghold – “Orcs have no place here” [Campaign 6 – Mission 1]

I can't seem to complete this mission, even though I understand that it shouldn't be that difficult. I capture the castle, free the first goblin slaves, and then I encounter Karl for the first time. Soon after, they attack me: Karl from the East and someone else from the North. But I don't have an army... I can't defend myself... And it's the same every time I try again – I don't have enough time to build up an army... I have no idea what to do.