The Castle is the Order's headquarters in Strife. After you accomplish Macil's first few missions (freeing prisoners, destroying power crystal, and opening castle gate in sewers), the player along with a lot of Front troops attack the castle which is defended by Stalkers, Sentinels, Acolytes, Crusaders, and a couple of Reavers in buildings, plus some turrets. You are sent in to find and kill the Programmer, who controls the AI of the Acolytes. The Programmer is located in the Programmer's Keep at the back of the Castle. After you kill the Programmer the Castle serves as the new Front Base.