What is the importance of the Castilla in the novel, Hornblower and the Atropos?

The Castilla is a Spanish frigate, or ship of war—a rare survivor of Trafalgar. She carries forty-four guns and throws three times the weight of metal as Atropos in a broadside. Castilla is berthed at Cartagena when Atropos passes by and pursues the English ship for a few hundred nautical miles until eventually engaging her in combat, aided by HMS Nightingale. The chase and combat with Castilla consume the final three chapters of the novel. Castilla bears forty-four 18-pounder long guns, making her the equivalent of the largest British fifth-rate.