The disappointing thing about illusions is that no matter how thorough and clever, they fail to deceive one person - the caster.
Even though the guards patrol and see him stately in his royal diadem, it does not change the fact that he is barefoot and bedraggled among the wreckage of his cell.
He deceives those who scorn his fall, while certain people see what has become of him. He knows it is petty, but his rankled soul is mollified - if only for a moment - by knowing that it distresses them to see what their once proud son - brother - has become.