Ranger – “Defense” [Campaign 5, Mission 3]

That's how I played through it. I captured the Castle, left 2/3 of the troops in the city and moved west along the river, then south, landed on the shore and requested Necropolis units from the Elven city. The main thing is to hold out for the first few weeks - then the 2nd hero levels up and defending becomes much easier.
And one more thing - try to build up the 1st city as quickly as possible and develop the Avenger skill for the hero in it; with 4 sworn enemies, defending becomes much easier. The Necropolis units of the east will also sail in there, but they can be intercepted by the main hero - if the 2nd city is built up properly and the army is no longer small. I wouldn't recommend taking troops out of the 1st city, at least for the first month.
And once you build up the 2nd city - you can move further along the path, toward the prison (in the southwest) and toward the Necropolis (in the southeast).