In ‘The Castle’ Muir writes:Our only enemy was gold’! This is true because

Explanation

The poem tells the story of the unexpected defeat of an empire. In the beginning, the narrator builds an atmospheres of confidence within the castle before the invasion, one that would lead the reader to assume the soldiers who were at the castle never suspected that such a fall was possible. However, the soldiers were betrayed by one of their own, the warder, who let the enemy through a small gate after being bribed. 

Hence the last two stanza reads;

''Our only enemy was gold,
And we had no arms to fight it with.''