Before heading south to the fort, make sure to visit your town and recruit all the units you can afford.
Also, by now you might be able to build the unique building, Angel Watch. This will complete the quest "Improve the Mausoleum" and reward you 3000 gold for your perseverance.
The Angel Watch fills the Guardian Angel Faction ability gauge to rank 1 at the start of each combat.
Remember to own a mine or dwelling you need to capture the fort or city controlling the area.
Head south, cross the water to get to the Haven fort.
Move into it to attack it.
The central tower has its own resistance. It cannot shot, but it can be garrisoned by one creature from the defender's army.
Creatures gain a special effect while they are in the central tower called, it negates range penalty and gives a small bonus to ranged damage. This effect also applies a minimum range to the garrisoned creatures. Enemy creatures next to the central tower can't be targeted from inside the tower. When the central tower is damaged, the creature stack in the tower is damaged proportionately.
Capturing the enemy fort will complete the quest "Expand your territory" if you have earlier occupied the mines and the saw mill. Capturing the fort allows those mines to be under your control permanently. You also get a nice reward including 5 ore, 5 wood, and 5 blood crystal.
Enter the fort and recruit any units there.
Having captured the fort, you also completed the objective from Wolves at the Barndoor. Your next objective in this quest involves the spirit gate.
Note the spirit gate. Spirit gates like this one will teleport to distant places. Click on the spirit gate and see where it leads.