Based on the original Roman fort, the medieval defences are a magnificent tourist attraction after being saved from demolition by over zealous nineteenth century town planners!
Visiting the Medieval Defences of York
York is well geared up as a tourist city. Once you are in the centre of the city you can walk to all the major attractions and everything is well signposted. There are obviously plenty of facilities and distractions too. There is a link to a wall walk guide in the References section and that is probably the best place to start, with a circuit of the walls. Each of the gates can then bu visited and finally Clifford’s Tower, the original castle keep. The only time the defences were actually put to use was a couple of hundred years after they became outmoded, in the English Civil War.
My Experience
We have stayed in the city many times over the years to explore its various historical treasures. We usually stay at the Hilton which is about 30 seconds walk from Clifford’s Tower. The Medieval Defences of York are still well preserved, or at least what’s left after the barbarism of the nineteenth century planners who destroyed all the barbicans, some of the bars and sections of wall. The original castle has entirely gone other than the keep, some water defences and a small section of wall.
Medieval Defences of York Photo Gallery
Mickelgate Bar 
City Walls 
Roman tower 
City Walls 
City Walls
These next photos were taken in the dim and distant past of 2008 and they were not taken in RAW so these are pretty much unedited jpgs. I’d like to think my own skill in photography has moved on since then too so these may not be ideal pictures. Sorry!
History
The city or ‘bar’ walls of York are the most complete example of medieval city walls still standing in England today. Beneath the medieval stonework lie the remains of earlier walls dating as far back as the Roman period.
The Roman walls survived into the 9th century when, in AD 866, York was invaded by the Danish Vikings. The Vikings buried the existing Roman wall under an earth bank and topped with a palisade – a tall fence of pointed wooden stakes.
The wooden palisade was replaced in the 13th and 14th centuries with the stone wall we see today.
The medieval city walls originally included 4 main gates or ‘bars’ (Bootham Bar, Monk Bar, Walmgate Bar and Micklegate Bar), 6 postern or secondary gates and 44 intermediate towers. The defensive perimeter stretched over 2 miles encompassing the medieval city and castle.
By the late 18th century, however, the walls were no longer required as defences for the city and had fallen into disrepair. In 1800, the Corporation of York applied for an Act of Parliament to demolish them. In addition to the poor condition of the walls at the time, the narrow gateways of the bars were inconvenient and the walls themselves hindered the city’s expansion.
Many other cities, including London, were removing their outdated, medieval city walls at this time. In York, however, the city officials met with fierce and influential opposition and by the mid-nineteenth century the Corporation had been forced to back down.
Unfortunately, the call for preservation came too late for some parts of the walls – the barbicans at all but one of the gateways (Walmgate Bar) had been torn down along with 3 postern gates, 5 towers and 300 yards of the wall itself.
Since the mid-nineteenth century the walls have been restored and maintained for public access, including the planting of spring flowers on the old Viking embankment. Today the walls are a Scheduled Ancient Monument and a Grade 1 listed building.
References
There is a good summary on the defences of York at the History of York site here and the history above is copied from there.
There is a very comprehensive academic review of the defences here.
The Richard III Experience site here has some fantastic information here, going from the Roman through to Viking and Norman defences. It includes a guided walk here.