The makeshift forge is where Geralt finds either a dwarven blacksmith named Malcolm Stein (Scoia'tael path) or an Order Armorer (Order and Neutral path). Whichever one is there will be the one to make Raven's armour for the witcher. If the neutral path was chosen, Kalkstein will fashion Raven's armour for Geralt and this forge is used only to forge weapons and to sell items as per the Order path.
Additionally, if Geralt chooses the Scoia'tael path, then the blacksmith also functions as a merchant who buys and sells quite a large variety of goods.
The staff at the makeshift forge varies with Geralt's choice of path. If he chooses the Scoia'tael path, then the blacksmith is none other than Malcolm Stein, but if he chooses the Order path, then he is served by an Order armourer. On the neutral path, the forge is not the place to get armour, Kalkstein's lab is.