That is one of Torstan's maps that he did for a game "World of Rhune". He posted it on his website.

As far as I know he did not do a WIP here since it was a commissioned piece but if you take a look at some of his other maps you will see some of this style and he may have mentioned things about his process in those. Alternatively, he's active here so if you have a question ask away and he will probably come by and offer up whatever help he can.