Built by Edward III in the 1360s and named after his Queen, Philippa of Hainault, Queenborough was both circular and concentric, and surrounded by a moat, its layout anticipating the castles of Henry VIII by nearly 200 years. The outer wall was without flanking towers save for those on either side of the main entrance in the west. Opposite them on the eastern side was a postern gate. From the ground plan it is clear that an attacker, having forced the western gatehouse, would first be corralled within a line of walls with gaps in the middle and then have to go halfway round the outer court and enter another line of walls before reaching the main entrance to the inner ring or rotunda. While doing so, he would be brutally exposed to fire from the curtain walls and six flanking towers that formed the core of the castle. And if he penetrated the rotunda, he would find it compartmentalised and therefore impossible to take in a rush. 

This defensive system, which to Professor Brown has "all the simplicity of genius", was never put to the test. The importance of Queenborough and its new, unfortified town hardly survived Edward's death in 1377. In 1650 the castle was found to be obsolete. Soon after, it was demolished.

It was in search of this "castle in the air" that I set out for Queenborough on a fine summer's day. It is an attractive little town, its main street running down to The Swale, the channel between Sheppey and the mainland. On the south side of the High Street is a creek, full of boats of all sizes and descriptions, among them a small fishing fleet.

I had expected to find the site of the castle in the middle of town but the local council office directed me to the top of the High Street. There, between a car park and the railway station, is a gently sloping grassy mound, on top of which are benches and the remains of a 19th-century pump house, directly above the well which formed the core of Edward's fortress. The reason for the positioning of the castle became clear. It stood at the top of the creek, watching over a port that in 1368 became the centre of the wool trade along the coast from Gravesend to Winchelsea.

The modern town is aware of its illustrious past. A helpful notice board by the mound tells the story, and on the sea wall at the other end of the High Street the words "Welcome to Queenborough" are written below an inaccurate painting of the castle. But Edward's "cloud-capp'd towers" have dissolved into a scene of utter banality. That is why the place is so tantalising. With cars on one side, the railway on another and a nondescript industrial block on the third, the mind races to imagine what this final florescence of royal castle building in the Middle Ages must have once been like.