War took up a great deal of a king's time in the Middle Ages. Earls and barons were soldiers above all. Even bishops and abbots sometimes led their knights into battle.
Knights were soldiers who fought on horseback. They were the main fighting force in every army. A knight's main weapons were the lance (a long, thin spear) and the sword. It took many years of training to become a skilled knight.
A knight went into battle dressed in armour made from lots of iron rings (sometimes called chain-mail). He had an iron helmet, and he carried a long, narrow shield. A knight had to be strong to carry all that weight. So had his horse! War-horses were big, heavy animals. They could trot, but not gallop.
At first, Norman knights lived in the great lords' castles. But not long after 1066, the lords began to give away pieces of land to their knights. A knight paid rent for his land by doing guard duty at his lord's castle, and serving with his lord in the army.
Walter Robson: Medieval Britain; Oxford University Press, 1991/2000, page 10