Castilla was an Aragon-class unprotected cruiser of the Spanish Navy that fought in the Battle of Manila Bay during the Spanish–American War. She was built at Cadiz, Spain. Her construction as an armored corvette with a central battery ironclad design began in 1869. In 1870, her design was changed to that of an unprotected cruiser or wooden corvette, and, after political events delayed her construction. During the first two years of the Philippine Revolution in 1896–1897, Castilla patrolled to intercept contraband destined for the Philippine insurgents and supported Spanish Army forces fighting ashore in Cavite Province on Luzon. When the Spanish–American War broke out in April 1898, Castilla was part of the squadron of Rear Admiral Patricio Montojo y Pasarón in Manila Bay and was subsequently engaged and sunk in the Battle of Manila Bay.

An unprotected cruiser was a type of naval warship that was in use during the early 1870s Victorian or pre-dreadnought era. The name was meant to distinguish these ships from “protected cruisers”, which had become accepted in the 1880s. A protected cruiser did not have side armor on its hull like a battleship or “armored cruiser” but had only a curved armored deck built inside the ship — like an internal turtle shell — which prevented enemy fire penetrating through the ship down into the most critical areas such as machinery, boilers, and ammunition storage. An unprotected cruiser lacked even this level of internal protection. The definitions had some gray areas, because individual ships could be built with a protective deck that did not cover more than a small area of the ship, or was so thin as to be of little value. The same was true of the side armor on some armored cruisers. An unprotected cruiser was generally cheaper and less effective than a protected cruiser, while a protected cruiser was generally cheaper and less effective than an armored cruiser, with some exceptions in each case.