A castle warrior or castle serf was a landholder obliged to provide military services to the ispán or head of a royal castle district in the medieval Kingdom of Hungary. Castle warriors "formed a privileged, elite class that ruled over the mass of castle folk" from the establishment of the kingdom around 1000 AD. Due to the disintegration of the system of castle districts, many castle warriors became serfs working on the lands of private landholders in the 13th and 14th centuries; however, some of them were granted a full or "conditional noble" status.

The ispán or count was the leader of a castle district in the Kingdom of Hungary from the early 11th century. Most of them were also heads of the basic administrative units of the kingdom, called counties, and from the 13th century the latter function became dominant. The ispáns were appointed and dismissed by either the monarchs or a high-ranking royal official responsible for the administration of a larger territorial unit within the kingdom. They fulfilled administrative, judicial and military functions in one or more counties.