Answer: metaphor
In this excerpt, the comparison between a castle and a mule is an example of a metaphor. It is intended to provide more information on the mule. For example, this could mean that the mule was particularly strong and resilient, or that it was very valued. The answer could not be personification because nothing is being compared to a human. Moreover, it also could not be a simile as it is not a direct comparison with words such as "like." Finally, it is not a symbol as the mule, at least in this excerpt, does not stand for anything besides itself.