Many stone castles had a keep, the strong inner tower of a castle, also called a great tower. Stone keeps had walls that could be very thick, six metres in places at Dover. But apart from dropping stones or shooting arrows from the roof or windows, great towers gave defenders little chance to strike back. That weakness shaped what came next.

Attackers found other ways in. Stone keeps were vulnerable to catapults and to undermining. In undermining, attackers hacked at the outer face of the walls, placed wooden pit props under the overhanging masonry and set fire to them, as at the siege of Rochester Castle. During long sieges, as at Kenilworth Castle in 1266, starvation was the besiegers' best weapon.

So builders changed the plan. From the later 12th century castle building shifted from keeps to outer defences. Gatehouses, the most vulnerable point, often became mini-castles, and walls were made taller to stop missiles being flung over them and to give extra velocity to missiles aimed at attackers. Some gatehouses had murder holes, holes in the roof of a passage through which missiles could be dropped on attackers.

By about 1250 Dover Castle had inner and outer circuits of towered walls around its keep, each line of defence supporting the others, which made it the first concentric fortress in western Europe. A concentric castle had two rings of walls, one inside the other, and the inner wall was higher, so archers could shoot over the heads of soldiers on the outer wall.

Dover's walls were tested. In the great siege of Dover Castle in 1216 the castle held firm for four months against French forces, hoping to secure it for Prince Louis of France, who had invaded England at the invitation of King John's rebellious barons. The defenders were 140 knights and perhaps a thousand sergeants, the medieval name for any fully-equipped soldier.

Weapons mattered too. The longbow, ideally made of a single bough of yew, had a range of well over 200 metres, and at shorter ranges its needle-pointed arrows could pierce armour. Crossbows were powerful but shot slowly, and their most experienced users were often foreign mercenaries. Then came gunpowder. When gunpowder and cannon came, castle defences could be damaged more easily, so castles mattered less.

Words to know

• undermining: digging beneath
• masonry: cut stonework
• mercenaries: paid soldiers
• sergeants: equipped soldiers