The Multangular Tower in the Museum Gardens is the most noticeable and intact structure remaining from the Roman walls. It was constructed as part of a series of eight similar defensive towers, built on the orders of the Roman Emperor Septimius Severus, who lived in Eboracum, as Roman York was known, from 209 to 211 AD. It has ten sides and is almost 30 feet (9.1 m) tall. The lower courses are original Roman stonework, though the upper course with arrowslits is a later medieval addition.
The Multangular Tower and the other seven similar towers were, however, a late addition to the walls. The original walls were built around 71 AD, when the Romans erected a fort (castra) occupying about 50 acres or 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, as follows:
- a section (the west corner, including the Multangular Tower) in the Museum Gardens
- the north-west and north-east sections between Bootham Bar and Monk Bar
- a further stretch between Monk Bar and the Merchant Taylors' Hall, at the end of which the lower courses of the east corner of the Roman wall can be seen on the city-centre side of the existing wall.
The line of the rest of the Roman wall went south-west from the east corner, crossing the via principalis of the fortress where King's Square is now located. The south corner was in what is now Feasegate, and from here the wall continued northwest to the west corner. The point where the wall crossed the via praetoria is marked by a plaque in St Helen's Square near the Mansion House.
Source & More Information: Wikipedia, York city walls, http://en.wikipedia.org/wiki/York_city_walls