Koskun keep is found east of An-Havva. You enter through the green arrow. The red arrow leads to Sengir Castle.
Locations of interest Edit
1 - Entrance to Plains of Rath
2 - A tower that can be used as a safe zone to rest
3 - An orcish mine. You can go in, getting out is harder
4/5 - A ladder leading to the cellar at the inn in An-Havva