The castle of Coucy was originally built as a wooden castle in the 9th and 10th century. In 1116 Enguerrand I, de Boves received the castle of Coucy as a feudal tenure and founded the powerful, dynasty of the barons of Coucy, which existed until 1397. Related  to several European royal houses, the barons continued to acquire property.

Enguerrand III was reputed to be the most enigmatic personality of this dynasty. He owned the castle of Coucy in its medieval heydey. The tremendous defence facilities were erected from 1223-25 and had the highest keep ever built in France.  It was meant to serve as a manifestation of his power. After his marriage to the English King’s granddaughter, his domain was considerably increased, and so after the death of Louis VIII, the Lion,in 1226, he claimed the French throne. Only thanks to the prudence of the later sovereign Blanca of Castile, Enguerrand’s plans were thwarted, due to this he formulated his famous arrogant motto:"I am neither king nor prince nor duke, not even a count, but I rule Coucy".

During the Hundred Years War the construction withstood the attacks of English troops in 1339.

Important extensional and reconstructional work was started in the late 14th century by Enguerrand VII.

With the death of the oldest, childless daughter of Enguerrand VII, who left Coucy to King Charles VI’s brother Louis, Duke of Orléans, the House of Coucy finally died out around 1400.

Enguerrand’s work was completed by Louis. He extended Coucy, which also played an important military part in the line of defence north of Paris, and made it into one of the most splendid residences in France.

After Louis was murdered in 1407 Coucy became an object of conflict and was besieged in 1411 and in 1413. After restless decades the castle was finally conquered by royal troops in 1487 following another siege and was handed over to Charles VIII’s son: Louis, Duke of Orléans, who was crowned in 1498.

During the Religious Wars Coucy was attacked by the Huguenots in 1567 and then occupied by the Ligeurs.

When, in 1652 the governor of Coucy refused to hand over the castle to the marshal, acting on behalf of Mazarin, the castle’s gates and outer bailey as well as the keep’s chemise and its vault were blown up. Coucy was used as a quarry, later as a prison and an old people’s home. In 1856 Viollet-le-Duc commenced its restauration.

Before their retreat on the 27th March , 1917 German troops blew up the keep with 28 tons of dynamite.

The well fortified town of Coucy is situated on a limestone platform 60m above the river Ailette. The huge fortress formerly stood at the northern end of the town, separated from it by a 25 m wide trench, and projected out into the valley. 

Through the splendid double gatehouse "Maître Odon" dating from 1225, destroyed in 1652, one reached the enormous outer bailey with its 9 closely situated 9m diameter round towers. Three of these towers have been completely destroyed and in the outer bailey only the well, the foundation of a chapel and 6 bases of portal columns are still visible.

The viaduct-type drawbridge consisted of several parts.  It rested on a wall of columns and spanned over the 22m wide ditch to the ward’s gate, and was protected by 3 portcullis and a murder hole. Above the tower’s wall walk there used to be a residential building with castle guards and loop-holes on either side.  This was built in 1538 and destroyed in 1653.

The trapezium-shaped ward measured 111m at the front by 51 x 70 x 105m and had a surface area that far surpassed all other royal castles. Even the flanking, 4-storeyed round corner towers were 35m high and up to 20m of diameter.  These were larger than Philip’s keeps, although they were similarly equipped with several fireplaces, lavatories and ribbed vaults in the lower floor and ground floor, while the upper storeys had wooden ceilings.

The storey wise staggered embrasures made possible a perfect defence. The wall thickness increased on the attacker’s side. Four metres below the top of the wall was a wreath of corbel stones for hoarding. Another 4m beneath this was the wall walk which ran the length of the curtain wall, after it was raised around 1400. In the 13th century the length of the curtain wall was reinforced with spurs and arches on the the courtyard side to avoid dead angles. Over the arcades lay the wall walk on the then vacant eastern length of the curtain wall.

Of the north-eastern tower only a stump remains. The circular cellar and ground floors of the others with attractive details and the precise masonry technique are partly preserved. Only a few remains of the 2-storey servants building along the eastern length of the curtain wall (14th century ) have survived.

The buildings within the castle’s courtyard were radically modernized between 1380-89, espescially the residential "Salle des Preuses" and the hall "Salle des Preux", which was connected via 2 portals to a chapel (1235-40) which was situated higher. The well preserved cellar hall was segmented with 9 arch openings and the hall had 2 pointed tunnel roofs. Considering the size of the cellar, the previous building had unusual measurements. The low-ceilinged hall on the ground floor was vaulted in the 14th century with centre columns and used as a wine cellar. On the 1st floor beneath an enormous, open pointed tunnel vault, was one of the largest great halls of the Middle Ages.  It had a platform at the northern end and a large surface tracery window in the southern corbie-step gable. There were 2 colossal fireplaces and 2 splendid windows beneath the lucarnes. Still to be seen today on the western wall are 4 of the 9 original niches with protective stone roofs for the 9 heroes,  Above them on the roof, at a height of 10m, is a wreath-shaped moulding.

A 4,92m wide and 20m high tremendous semi-circular chemise surrounded the keep, the most colossal medieval round tower in France, projecting out on the attacker’s side as the main defence and reached by a carefully constructed draw bridge construction. The entrance gate, decorated with a pointed tympanon with a relief of a knight overpowering a lion, leads under the portcullis and the drawbridge winding room to a vaulted corridor. Connecting hallways lead to a 212-step spiral staircase, which led up to the defence platform on the right and to a lavatory on the left. A door, which could be secured by means of a drawbar, led to the hall on the ground floor, which contained a fireplace, an oven and a 64m deep well.

All three twelve cornered hall-floors were vaulted with twelve-armed ribbed vaults and a closing ring. The physical extension and division of the walls were made by 12 gothic lunettes, as tall as the room, which were between the spurs in the vaulted niches. Only on the ground floor were two lower niches in two rows on top of each other. Completing the details were the round columns in the intersection points of the wall surfaces which carried capitals with kneeling painted stone figures on their imposts.  These were also featured on all the other upper floors.

The first floor had a fireplace and lavatory at the back walls of 3 niches, while stone staircases led to the highly placed windows. An escape passage led to a poterne, which was connected by a mobile foot bridge to the walkway on the surrounding wall.

On the 2nd floor was a hall, extended by 11 galleries, with two windows which scarcely illuminated it.