The village is a death trap, a swamp where many predators reside. Now if only there's a place where I can build and gather my faction there. Guessing Kirlic is no safe place either, some dark time ahead.
Well, technically, you already have a rather save spot and means to build a fort, or maybe even a castle, overlooking the Arenfield and the forrest. Just, sadly, not implemented in the game.
Your mercenary group camp is a perfect place for it. Protected by the elven magic and with one easy access route to it, beneath a huge hill with a small cave and water supply inside and also near a river. I would say it's a perfect place to build at least a fort on it.
Resources are not really a problem here.
If you want to build a wooden fort, like romans usually did, you have resources right beside you. With Tia, your mercenaries and Rumah you could gather enough wood in no time to do so.
If you would like to build a fort made of stone, then you can easily gather a lot of stone as the Arenfield's iron mine is already producing it, but as wastes nobody is even talking about. There are also quite tall and rocky hills right to the village itself just waiting to be mined for resources in greater quantity than just to expand Emily's house. And there is also Rumah, where locals are mining copper, which means also stone wastes laying around. The minior problem here, however, would be transport. While carts wouldn't be too much of a problem to make, especially with the help of local carpenter, it would surely cost a lot to hire people to push it so far away and catch the attention of the locals, especially cult and Giron.
As for the question how long it will take, I'll say it depends. Because there won't be too much problem with labour too, as you have Rumah and mercenaries to do all the work after they gather all the necessary resources. Of course wooden fort would take probably not more than a month to be build, while stone one would take many of them, if not years, depending how much people would work on it. Not to mention the presence of experience engineer (someone from Kirlic university perhaps?) to help you with the more problematic parts of the keep, gatehouse and towers.