York has, since Roman times, been defended by walls of one form or another. To this day, substantial portions of the wall remain and York has more miles of intact wall than any other city in England. The original walls were built around 71 AD, when the Romans erected a fort (castra), occupying 21.5 hectares near the banks of the river Ouse. The rectangle of walls was built as part of the fort's defences. The foundations and the line of about half of these Roman walls form part of the existing walls.
When walking on the walls, one gets stunning views around the city, including the very impressive York Minster (built between 1220 and 1472).