What was feudalism? How were castles involved? How did this affect military service?

Feudalism was a political and economic system of Europe from the 9th to about the 15th century, based on the holding of all land in fief or fee and the resulting relation of lord to vassal and characterized by homage, legal and military service of tenants, and forfeiture. Castles showed power, and offered protection. This gave the lord a military.