Pretty self explanatory this one. Basically conquer 2 fortresses. There are 4 in the game and are pretty noticeable on the map.
There is Le Fleur's, Yuma, Noore and the one and only Pagan Min's to do and to be fair you get decent XP from each one. You can either do them in Coop which is fun or you can wait until you have decided the fate for every character as this will weaken each one meaning less guys inside.
I wouldnt recommend storming a fortress alone without deciding the fate of the character who's stronghold it is as they are actually quite hard even on easy!!!
Anyway conquer 2 for this achievement and do all 4 for more XP which will go towards skill points.