Please answer a question about the following article about Napoleon:

The 5th Regiment was sent to intercept him and made contact just south of Grenoble on March 7, 1815. Napoleon approached the regiment alone, dismounted his horse and, when he was within gunshot range, shouted to the soldiers, "Here I am. Kill your Emperor, if you wish." The soldiers quickly responded with, "Vive L'Empereur!" Ney, who had boasted to the restored Bourbon king, Louis XVIII, that he would bring Napoleon to Paris in an iron cage, affectionately kissed his former emperor and forgot his oath of allegiance to the Bourbon monarch. The two then marched together towards Paris with a growing army. The unpopular Louis XVIII fled to Belgium after realizing he had little political support. On March 13, the powers at the Congress of Vienna declared Napoleon an outlaw. Four days later, Great Britain, Russia, Austria, and Prussia each pledged to put 150,000 men into the field to end his rule.

Ney had told Louis XVIII that he would carry Napoleon to Paris in what kind of container?
an iron cage