The Castle is an unlikely last stop in the brilliant career of three-star General Irwin. Court-martialed and stripped of his rank, Irwin has been sentenced to the maximum security military prison, which is run with an iron fist by its warden, Colonel Winter. Winter can't help but respect the once-legendary general, but it isn't long before that respect turns to resentment and then open hostility as Irwin continually confronts the warden on his methods. Setting out to break Irwin by whatever means necessary, the colonel's tactics only fuel Irwin's defiance and cause the other prisoners to rally behind the general in his new mission: to seize control of the prison and remove Winter from his command. The men imprisoned in The Castle have been told that they are no longer soldiers... but they are about to prove that they can still fight a war.

When a powerful Army General is imprisoned in the country's only maximum-security stockade, he systematically garners loyalty among the men and raises an army to take over the prison.