Château Gaillard (French pronunciation: [ʃɑto ɡajaʁ]) is a medieval castle ruin overlooking the River Seine above the commune of Les Andelys, in the French department of Eure, in Normandy. It is located some 95 kilometres (59 mi) north-west of Paris and 40 kilometres (25 mi) from Rouen. Construction began in 1196 under the auspices of Richard the Lionheart, who was simultaneously King of England and feudal Duke of Normandy. The castle was expensive to build, but the majority of the work was done in an unusually short period of time. It took just two years and, at the same time, the town of Petit Andely was constructed. Château Gaillard has a complex and advanced design, and uses early principles of concentric fortification; it was also one of the earliest European castles to use machicolations. The castle consists of three enclosures separated by dry moats, with a keep in the inner enclosure.

Château Gaillard was captured in 1204 by the king of France, Philip II, after a lengthy siege. In the mid-14th century, the castle was the residence of the exiled David II of Scotland. The castle changed hands several times in the Hundred Years' War, but in 1449 the French king captured Château Gaillard from the English king definitively, and from then on it remained in French ownership. Henry IV of France ordered the demolition of Château Gaillard in 1599; although it was in ruins at the time, it was felt to be a threat to the security of the local population. The castle ruins are listed as a monument historique by the French Ministry of Culture. The inner bailey is open to the public from March to November, and the outer baileys are open all year.

Richard the Lionheart inherited Normandy from his father, Henry II, in 1189 when he ascended the throne of England. There was a rivalry between the Capetians and the Plantagenets, Richard as the Plantagenet king of England was more powerful than the Capetian king of France, despite the fact that Richard was a vassal of the French king and paid homage for his lands in the country. From 1190 to 1192, Richard the Lionheart was on the Third Crusade. He was joined by Philip II of France as each was wary that the other might invade his territory in his absence. Richard was captured and imprisoned on the return journey to England, and he was not released until 4 February 1194. In Richard's absence, his brother John revolted with the aid of Philip; amongst Philip's conquests in the period of Richard's imprisonment was the Norman Vexin and a few towns around such as Le Vaudreuil, Verneuil and Évreux. It took Richard until 1198 to reconquer a part of it.

Perched high above the River Seine, an important transport route, the site of Château Gaillard, in the manor of Andeli, was identified as a naturally defensible position. In the valley below the site was the town of Grand Andely. Under the terms of the Treaty of Louviers (January 1196) between Richard and Philip II neither king was allowed to fortify the site; despite this, Richard intended to build a castle at Andeli. Its purpose was to protect the duchy of Normandy from Philip II—it helped fill a gap in the Norman defences left by the fall of Château de Gisors and above all Château de Gaillon, a castle which belonged to Philip and used as an advanced French fortification to block the Seine valley—and to act as a base from which Richard could launch his campaign to take back the Norman Vexin from French control. Indeed, Les Andelys is located just in front of Gaillon on the other side of the Seine valley. Richard tried to obtain the manor through negotiation. Walter de Coutances, Archbishop of Rouen, was reluctant to sell the manor as it was one of the diocese's most profitable, and other lands belonging to the diocese had recently been damaged by war. When Philip besieged Aumale in northern Normandy, on the border with Picardy, Richard grew tired of waiting and seized the manor, although the act was opposed by the Church.

In an attempt to get Pope Celestine III to intercede, Walter de Coutances left for Rome in November 1196. Richard sent a delegation to represent him in Rome. One of the parties, Richard's Lord Chancellor William Longchamp (who was also Bishop of Ely), died during the journey, although the rest, including Philip of Poitou, Bishop of Durham, and Guillaume de Ruffière, Bishop of Lisieux, arrived in Rome. Walter de Coutances meanwhile issued an interdict against the duchy of Normandy which prohibited church services from being performed in the region. Roger of Howden detailed "the unburied bodies of the dead lying in the streets and square of the cities of Normandy". Construction began with the interdict hanging over Normandy, but it was later repealed in April 1197 by Celestine, after Richard made gifts of land to Walter de Coutances and the diocese of Rouen, including two manors and the prosperous port of Dieppe. The site of Château Gaillard had not been fortified before, and the town of Petit Andely was constructed at the same time; together with the historical Grand Andely, the two are known as Les Andelys. The castle sits on a high limestone promontory, 90 m above Les Andelys and overlooking a bend in the River Seine. The castle was connected with Les Andelys through a series of contemporary outworks.

During King Richard's reign, the Crown's expenditure on castles declined from the levels spent by Henry II, Richard's father, although this has been attributed to a concentration of resources on Richard's war with the king of France. However, the work at Château Gaillard cost an estimated £12,000 between 1196 and 1198. Richard only spent an estimated £7,000 on castles in England during his reign, similar to his father Henry II. The Pipe rolls for the construction of Château Gaillard contain the earliest details of how work was organised in castle building and what activities were involved. Amongst those workmen mentioned in the rolls are miners, stone cutters, quarrymen, masons, lime workers, carpenters, smiths, hodmen, water carriers, soldiers to guard the workers, diggers who cut the ditch surrounding the castle, and carters who transported the raw materials to the castle. A master mason is omitted, and military historian Allen Brown has suggested that it may be because Richard himself was the overall architect; this is supported by the interest Richard showed in the work through his frequent presence.

Not only was the castle built at considerable expense, but it was built relatively rapidly; construction of large stone castles often took the better part of a decade; for instance, the work at Dover Castle took place between 1179 and 1191 (at a cost of £7,000). Richard was present during part of the construction to ensure construction proceeded at a rate he was happy with. According to William of Newburgh, in May 1198 Richard and the labourers working on the castle were drenched in a "rain of blood". While some of his advisers thought the rain was an evil omen, Richard was undeterred:

the king was not moved by this to slacken one whit the pace of work, in which he took such keen pleasure that, unless I am mistaken, even if an angel had descended from heaven to urge its abandonment he would have been roundly cursed.