Historians know a surprising amount about daily life in one castle, because the paperwork survived. We have unique insights into the household of Countess Joan de Valence, who owned Goodrich Castle in the late 13th century, thanks to the unusual survival of some of her accounts. Even so, the record has gaps, because working out who stayed where at Goodrich Castle involves examining the ruins and the medieval documents, comparison with other sites, and ultimately guesswork. Castles were places where people lived and worked, not only forts, and the accounts are what let us see that. The accounts give numbers. At Goodrich Castle, Countess Joan's cook Isaac of the Kitchen had to feed up to 200 people two meals a day. On Fridays and many other days people ate fish instead of meat, and the cooks at Goodrich Castle got through 24,000 herrings in three months, as well as oysters and salmon. Joan de Valence spent 40% of her income on food and drink. In winter, with no refrigeration, there was little fresh food, so at Goodrich cooks livened up salted meat or fish with strong-tasting sauces made with expensive foreign spices.

Meals were slow, and seating followed rank. There were two main meals a day in a great household, and they were not eaten quickly: lunch typically lasted two to three hours, and both meals together could take up to six hours. In the great hall, the host and the most important guests sat at the high table at the end furthest from the kitchen, and the rest of the household sat at long tables with benches in descending order of status. Owners were often elsewhere. Castle owners did not live in each castle all the time: the kings of England owned dozens of castles and could never have visited them all. In a time when the monarch, nobles and bishops were almost constantly on the move, King Henry III moved up to 80 times in a typical year. When Countess Joan arrived at Goodrich Castle in 1296, she came with a long procession of carts and waggons, over 50 horses and 194 followers. In peacetime a small castle might have a garrison of only a dozen soldiers or fewer, just enough to open the gate, work the portcullis and drawbridge and patrol the walls against stray robbers.

The people at the bottom of the household are harder to see, but the record gives a few facts. Lowlier castle servants slept anywhere they could find and in summer worked from 5.30am until about 7pm, with few days off and little pay, but they were given uniforms in their lord's colours and regular meals all year round. Medieval masons put a unique mark on the stones they cut so that they would be paid for them, and you can sometimes spot these marks in the stones of old buildings. In the summer of 1286, the busiest time at Harlech, nearly 950 men were employed: 227 masons, 115 quarriers, 30 smiths, 22 carpenters and 546 labourers. Young people worked in the household too. A page was a child from a wealthy family who learned good manners by working in a lord's household, and a squire was a knight in training who helped a knight.

Defences changed as attackers learned. 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. Stone keeps were vulnerable to catapults and to undermining, where 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. Rectangular keeps also had corners that could be undermined, so round keeps began to be built from about 1200. 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. 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. Murder holes were holes in the roof of a gatehouse passage through which missiles could be dropped on attackers.

The new design was 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. A force of 140 knights and perhaps a thousand sergeants, the medieval name for any fully-equipped soldier, defended the fortress. During long sieges, as at Kenilworth Castle in 1266, starvation was the besiegers' best weapon. Edward I's castles in north Wales show the idea at full strength. The concentric principle, reinforced by powerful gatehouses acting as keeps, was used for the chain of castles including Harlech and Beaumaris that Edward I built between 1277 and 1294 to control north Wales, which represent the high point of castle building in medieval Britain. Conwy and Caernarfon stand on rocky, narrow pieces of land with no space for a double wall, so these castles had extra towers and large gatehouses that could house small groups of soldiers instead. Edward had workers reroute a river when he built his castle at Conwy.

Weapons and armour kept changing too. Early knights wore mail, armour made of thousands of linked metal rings, and by the 1400s knights wore suits of plate armour. 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. As armour grew into full plate by the 15th century, knights seemed unstoppable, until powerful firearms capable of puncturing plate armour arrived. When gunpowder and cannon came, castle defences could be damaged more easily, so castles mattered less. Under the Tudors baronial castle-building ceased, but Henry VIII adopted new artillery technology for a chain of coastal gun forts, the first co-ordinated system of national defence since Roman times.

Weapons were kept at the Tower of London. An armoury had occupied buildings within the Tower for making and storing arms, armour and military equipment for as long as the Tower itself had existed, though it did not become the main royal and national arsenal until the 14th century. In July 1323 John Fleet was appointed keeper of the part of the king's wardrobe in the Tower, a key development that made the Tower a place of manufacture and storage of arms, armour and artillery. Some displays were not what they claimed: weapons said to have been taken from the Spanish Armada in 1588 mostly had no Spanish connections. The Line of Kings was a row of figures representing the kings of England, shown on life-sized wooden horses wearing what was said to be their personal armour. In the mid-15th century the Office of Ordnance and the Office of Armoury replaced the Privy Wardrobe: the Armoury concentrated on armour and edged weapons, and the Ordnance on cannon, handguns and the more traditional bow and arrow.

When the country became more peaceful, castles were no longer needed for defence: some were used as centres of administration where rents and taxes were collected, some held courts of law, and others fell into ruins. Kidwelly Castle was used as a court until 1609. In the English Civil War long abandoned castles were readied for war and, even against cannon they were never designed to defend against, many survived long sieges; however, many were slighted, or demolished, like Kenilworth, Scarborough and Helmsley, leaving ruined shells. Some castles left almost no trace: there are more than 700 early castle sites in England that are harder to spot than famous stone castles, most of them earthwork castles built from timber in the decades after the Norman Conquest in 1066, and the timber has long since rotted away. Many Englishmen profited from ransoms and plunder in the Hundred Years War, and the proceeds helped pay for the building of castles such as Farleigh Hungerford in Somerset.

Ruins found new admirers when, in the 18th and 19th centuries, Romanticism appreciated castle ruins for their history and beauty. In 1838 the cost of visiting the Tower Armouries was cut from three shillings to one shilling and lowered again the following year to six pence, and as a result visitor numbers rose from 10,500 in 1837 to 80,000 in 1839. Old castles also went back to work: after the fall of France in June 1940, when invasion seemed imminent, old castles and forts were re-commissioned as command centres, like the Secret Wartime Tunnels at Dover Castle, or to house guns to defend the coast, like Pendennis Castle and Hurst Castle. At Pevensey Castle, Second World War machine-gun posts were built into the walls and camouflaged with 'Roman' masonry. On 27 April 2023 Cadw completed a three-year, £5 million conservation project at Caernarfon Castle's King's Gate, opening sections of the castle unseen by the public for centuries.

Words to know

• garrison: castle soldiers
• slighted: demolished
• arsenal: weapons store
• mercenaries: paid soldiers
• Romanticism: art movement