Now do not be alarmed - this is not, so far as I am aware, a precaution in case Scotland breaks away from the Union and sends raiding parties down into Yorkshire as happened after Bannockburn - but rather an important piece of conservation work.
The City Walls are one of the great features of York, and probably the finest example remaining in England. Other cities have them - Chester a similarly complete circuit, significant remains in, amongst others, Oxford, Southampton, Canterbury, Winchester, Exeter, Lincoln and, if you know where to look in the City of London - but York’s magnesian limestone walls atop their embankment and the gateways or bars add a unique dimension to the city.
There is a well illustrated and informative online walking tour of the circuit at Walking trip around York City Walls history walk towers turrets barbican
Wear and tear takes its toll and there are now plans to repair a mural tower near the Old Baile at the southern point on the south-western section of the walls. To do that involves removing eighteenth century filling and affords the opportunity to examine how the walls evolved. The Old Baile is the motte of a second castle created by King William I to face the much better known one now topped by Clifford’s Tower on the other bank of the Ouse. Later it was given to the Archbishops of York, and their involvement in rebuilding the city fortifications at this point is alluded to in this article from the MailOnline. It can be seen at History of York's medieval city walls to be uncovered in excavation