The Britain Express website tells us:
The Romans erected the first walls around Canterbury between 270 and 290 AD. Very little of those Roman walls remain. The walls we see today are medieval. The medieval walls surounded the entire city of Canterbury and were pierced by 8 gates, West Gate, North Gate, Quenin Gate, Burgate, Newingate, Riding Gate, Worth Gate, and London Gate. Of these, only West Gate remains.
West Gate Tower was erected by Archbishop Sudbury in 1380. It did nothing to increase his popularity; he was murdered by rioting peasants the following year. The EWest Gate is historically important as it represents one of the first defensive structures built with the use of gunpowder and artillery in mind. It uses keyhole gunports to create opportunities for cannon fire from a well defended position.
The largest of the medieval gates was Riding Gate, which took traffic from the Dover road.
Several towers remain from the medieval fortifications. The most imposing of these is West Gate, but there are others in varying stages of repair, including Whitecross Tower, near Dane John Mound. whitecross takes its name from a white stone cross set into the exterior stonework. This cross is in memory of Protestants burned at the stake during the dark years of the English Reformation at nearby Martyrs Field.
Near Burgate, there is another tower, now converted to use as a chapel. At the south end of the car park near Burgate is a stretch of Roman wall incorporated into the medieval stonework. Look for herringbone pattern stonework and rounded boulders and flints. The outline of Roman Queningate (c. 270-290 AD) can be seen blocked up near the current entry to the cathedral precinct. This gate was blocked up in 1492/3.
Between Queningate and North Gate are four square towers, erected by Prior Chillenden between 1390 and 1396.
The medieval walls form a rough oval about 3000 yards circumference. Note the use of keyhole gun ports at several locations along the wall. These are possibbly the work of Henry Yevele, the master mason responsible for much of Bell Harry Tower at Canterbury Cathedral.
There wre originally 21 mural towers set into the walls. Of these 16 remain.
The best surviving section of the Roman wall is set into the rear of the Church of St Mary, Northgate. Here the walls still stand to 16 feet and are capped with original crenellations. There is a very well preserved section of Roman walling on St Radigund's Street, near the site of North Gate.
The city walls of Canterbury are among the best preserved in the country, and well worth a wander.
The Canterbury Buildings website tells us about the walls:
Canterbury was surrounded by a wall in Roman times. Traces survive here and there. A fragment of the Roman Queningate can be seen in the city wall opposite St. Augustine's Great Gate, and further up, nearer Burgate, the Roman foundation of the wall is visible.
The walls are mentioned in several Anglo-Saxon documents. In 1011 the Danes succeeded in breaking into the city, slaughtering the inhabitants, and tossing them over the walls.
It has not yet been established whether the Roman and Saxon walls ran altogether on the same line as the later medieval walls, but about 1100 A.D. the city fortifications included the same area as they did to the end of the 18th century.
There were six gates in use in medieval times:-Northgate, Burgate, Newingate, Ridingate, Worthgate and Westgate. Later another came into existence, Wincheap Gate. The walls were frequently rebuilt and reconstructed but never called upon to withstand any real siege after 1011, though the city represented an important strongpoint in the system of national defence.
The Royal government was always attentive to the condition of Canterbury's walls, and whenever they started to become ruinous a peremptory note from the Crown would order their restoration.
The continuation of Chaucer's Canterbury Tales tells how when the Pilgrims reached Canterbury the Knight took his son, the Squire, to view the City Walls.
Round 1800 wholesale destruction of the city's historical monuments was allowed to start, due mainly to street-widening programmes, and before it was arrested by an awakening interest a century ago in medieval antiquities, five of the city gates and half of the walls had gone.
The length of the wall was 1.5 miles. It was strengthened by 21 Watch Towers, most of which survive, though embedded in houses. Half of the wall itself still exists, the best visible stretches being at the Dane John and Broad Street. Much of the City Ditch is now built upon. This process was far advanced by the 17th century when it was lamented that the military value of the wall was thus nullified. Even in the 12th century a certain amount of squatting in the Ditch had taken place.
The Kent Council website has a PDF document of which the following is an extract:
The City walls are of Roman origin and more than half the medieval circuit survives. They were remodelled in the late 14th century to enable the use of gunpowder weapons in their defence and in this respect are among the earliest examples in the country.
The Roman town wall of Durovernum was built towards the end of the 3rd century AD and had at least one internal tower. Traces of Roman gates are recorded at Queningate, Riding Gate and Worth Gate and recent archaeological excavations have increased knowledge of the Roman fabric. The medieval wall followed the same alignment, a roughly oval circuit c.3000 yards in circumference. The ditch is mentioned in the Domesday Book and the walls were said to be in reasonable repair in c.1140. Repairs were carried out by the Crown later in the 12th century and again c.1290-1320. In 1363 a commission of enquiry described the walls as mostly fallen through age and the ditches obstructed. Work of renewal began by 1378. Archbishop Sudbury had begun the entire rebuilding of the West Gate in 1380 and in the 1390’s work commenced on the towers flanking the River Stour.
The master mason/'architect', Henry Yevele, is recorded as having a role in the construction of the defences and it may be to him that the use of 'keyhole' gun-ports can be attributed. Newingate/St George's Gate was built in c.1470 and may have imitated the West Gate since it had two circular towers. Many of the wall towers were repaired in the 15th and 16th centuries.
Most of the west side of the circuit, however, was demolished in 1648. The River Stour towers and parts of Burgate and St. George's Gate were demolished in 1769-92.
A section of Roman city wall survives to a height of c.16 feet and is capped by a continuous row of intact crenellations. This can be seen in the north wall of the Church of St Mary, Northgate. Standing medieval walling, often to parapet level now exists on the north-east, east and south sides but the ditch has been almost entirely filled in and in parts given over to car parking. There were, according to Hasted (1797-1801), twenty-one mural towers and seventeen of these remain. The towers are predominantly square on the north-east, and half-round to the east and south-east with a battered plinth. They are principally built in flint with ashlar quoins. Most contain key-hole gun-ports, one in each face. The wall-walk passed through the towers at first floor level. The West Gate has twin ashlar-faced drum towers with eighteen 'key-hole' gun-ports on three levels in addition to the traditional defensive measures in the gate passage. The gatehouse remains to full height.
Canterbury City walls are among the best preserved in England even though about one third of the circuit has been demolished. Of particular importance is the survival of a stretch of the Roman city wall to full height. The West Gate has considerable historic value being among the first documented defensive structures in the country to have been designed with the deployment of gunpowder artillery in mind. The 'keyhole' gun-ports are well designed and coordinated. They are of a textbook quality.
The surviving lengths of city wall are generally in good condition. In the 1950’s there was a programme of rebuilding a long stretch of wall on the east side which favoured visual effect rather than accuracy. A number of gun-ports were also restored clumsily. The West Gate appears to be in good condition. East of the site of North Gate in St. Radegunds Street there has been a recent exposure of early walling. Some of the towers on this side of the city are also in an un-restored state.