A stronghol is a strongly fortified defensive structure. The history of fortified buildings extends from antiquity to modern times. In Celtic Europe an example of a stronghold is the Hill fort, a large structure, with walls made of wooden stakes, and built on a steep hill. In medieval Europe, the castle was the most common type of stronghold and often the residence of the king or territorial lord. The Maginot Line is an example of a 20th century stronghold.