Henry Hall Brogden was imprisoned in this cell, having been charged with treason (being a Confederate spy). Lincoln essentially pardoned him, but did so by reducing his charges to entering Union lines without permission. Those charges amounted to an ex post facto application of the 100th Article of War, which had not been passed at the time of Brogden's capture. So Lincoln violated the Constitution (which forbids application of an ex post facto law) to save Brogden. Not clear why he couldn't just pardon him.