A castellan was the governor or caretaker of a castle or keep. The role was often part way between that of a majordomo and that of a military administrator — while a castellan was responsible for the work of a castle's domestic staff, as a majordomo was, the castellan was also often responsible for maintaining defences and protecting the castle's lands. This was particularly the case if there was no lord resident at the castle, or if the resident lord was frequently absent.

In France, castellans (known in French as Châtelains) who governed castles without resident nobles acquired considerable powers, and the position actually became a hereditary fiefdom. Also, there was at times a castellan among the Officers of the Kingdom of Jerusalem.

In the Kingdom of Poland and later Polish-Lithuanian Commonwealth the castellans were often considered to be subordinates of voivoides (with the exception of the Castellan of Cracow, which had a priviliged position, being Commonwealth capital untill 1596). Castellans whey were in charge of a part of the voivodship called castellany till 15th century, and from that time their domain was divided into provinces for Greater Castellans and powiat’s for Minor Castellans. Chancellors were district officials and had the right to attend sessions of Polish parliament, the Sejm.