Upon Rastislav's request, two brothers, Byzantine officials and missionaries Saints Cyril and Methodius came in 863. Cyril developed the first Slavic alphabet and translated the Gospel into the Old Church Slavonic language. Rastislav was also preoccupied with the security and administration of his state. Numerous fortified castles built throughout the country are dated to his reign and some of them (e.g., ''Dowina'', sometimes identified with Devín Castle) are also mentioned in connection with Rastislav by Frankish chronicles.