In the film 'Zulu', the besieged soldiers at Rorke's Drift sing a version of which Welsh song?

Zulu is a 1964 epic war film depicting the Battle of Rorke's Drift between the British Army and the Zulus in January 1879, during the Anglo-Zulu War. It depicts 150 British soldiers, many of whom were sick and wounded patients in a field hospital, who successfully held off a force of 4,000 Zulu warriors.

On the final morning, the Zulus approach to within several hundred yards and begin singing a war chant; the British respond by singing "Men of Harlech". In the final assault, just as it seems the Zulus will finally overwhelm the tired defenders, the British soldiers fall back to a small redoubt constructed out of mealie bags. With a reserve of soldiers hidden within the redoubt, they form into three ranks and fire volley after volley, inflicting heavy casualties; the Zulus withdraw. After a pause of three hours, the defenders are still recovering when the Zulus re-form again on the Oscarberg. Resigned to another assault, the British are astonished when the Zulus instead sing a song to honour the bravery of the defenders before departing.

The film ends with another narration by Richard Burton, listing the eleven defenders who received the Victoria Cross for the defence of Rorke's Drift, the most awarded to a regiment in a single action up to that time.