his land if William lost his crown.

_(b) Building Castles._--In the second place William built a castle in
every town of importance, which he garrisoned with his own men. The
most notable example of these castles is the Tower of London.

_(c) The Feudal Army._--In the third place, though the diffusion of
Norman landowners and of William's castles made a general revolt of
the English difficult, it did not make it impossible, and William took
care to have an army always ready to put down a revolt if it occurred.
No king in those days could have a constantly paid army, such as
exists in all European countries at the present day, because there was
not much money anywhere. Some men had land and some men had bodily
strength, and they bartered one for the other. The villein gave his
strength to plough and reap for his lord, in return for the land which
he held from him. The fighting man gave his strength to his lord, to
serve him with his horse and his spear, in return for the land which
he held from him. This system, which is known as feudal, had been
growing up in England before the Conquest, but it was perfected on the
Continent, and William brought it with him in its perfected shape. The
warrior who served on horseback was called a knight, and when a knight
received land from a lord on military tenure--that is to say, on
condition of military service--he was called the vassal of his lord.
When he became a vassal he knelt, and, placing his hands between those
of his lord, swore to be his man. This act was called doing homage.
The land which he received as sufficient to maintain him was called a
knight's fee. After this homage the vassal was bound to serve his
lord in arms, this service being the rent payable for his land. If the
vassal broke his oath and fought against his lord, he was regarded as
a traitor, or a betrayer of his trust, and could be turned out of his
land. The whole land of England being regarded as the king's, all land
was held from the king. Sometimes the knights held their fees directly
from the king and did homage to him. These knights were known as
tenants in chief (_in capite_), however small their estates might be.
Usually, however, the tenants in chief were large landowners, to whom
the king had granted vast estates; and these when they did homage
engaged not merely to fight for him in person, but to bring some
hundreds of knights with them. To enable them to do this they had to
give out portions o