The standard definition of castle, though, is the fortified residence of a feudal lord. Feudalism is the ownership of land for military service. This started in Mainland Europe in the 9th century as a response to the invasion of Europe by Vikings (known as Northmen or Normans in France), Arabs and Huns and to the breakdown of a strong central state with an army. Early earthwork castles date from this period.

These early earthworks were a simple bank of earth with a wooden fence, they were surrounded by a ditch which might (or might not) have water in it. These were not strong fortifications and some writers do not consider them castles. For these writers the presence of at least one tower is the deciding factor for a true castle.

The true castle, with a tower of some sort, dates to the end of the tenth century. Often the tower was a earth mound, known as a motte, with a wooden watch tower but stone towers were also built. The earliest stone castle is debatable but Langéais, in France, is still mainly standing as it approaches its 1000th birthday in the next few years, and Doué-la-Fontaine, also in France, is probable over a thousand years old.