The public gardens at Northernhay are the site of one of the most important surviving sections of Exeter's ancient city wall left. What might at first glance appear to be little more than a random jumble of different stones actually tells a story which goes back to the 2nd century Roman town of Isca Dumnoniorum. The section of wall also contains one of the rarest of British archaeological survivals: evidence of stone Anglo-Saxon fortifications.
The section of wall is easily reached by walking under the early 12th century Athelstan's Tower, the eastern tower of Rougemont Castle, and into Northernhay Gardens. Walk down the slope and the section of wall is on the right. (One of the tower's turrets is just visible to the far right in the photograph above.) This is the exterior face of the wall, the side which would've confronted anyone attacking the city from the north. The ground level here has been much-altered since the public gardens, the oldest in England, were first laid out in 1612 and again in 1664 following the restoration of Charles II.
The photograph right has been colour coded to show the remarkable stratified layers contained within the wall. The area highlighted in purple is Roman masonry from c200AD, now partially buried by the later landscaping. The natural ground level is about 16ft (5m) below the level of the path. The extremely durable purple volcanic trap used by the Roman builders was probably quarried nearby in either Rougemont or in Northernhay itself. The section highlighted in yellow is Norman and dates to just after the Norman Conquest of 1066. It is probably associated with the initial construction of Rougemont Castle in c1068. The two areas highlighted in green show where the wall has been patched and repaired in the medieval period. The repairs used the red Heavitree breccia quarried around Exeter from the mid 14th century onwards. The large section highlighted in blue is modern.
The section highlighted in red are the Anglo-Saxon crenellated fortifications. Sandwiched between the early Norman masonry above and the Roman masonry below, they are constructed from white Triassic sandstone probably quarried from East Devon. This is believed to be the only stretch of Saxon town wall to be seen anywhere in England. Relatively little is known about Exeter following the retreat of the Roman Empire from Britain in the early 5th century to the arrival of the Normans in 1066. Even less is known about the city wall itself which the Saxons inherited from their Roman predecessors. There is a tradition that King Athelstan overhauled the wall circuit around c920. The fortifications could date from then or perhaps they were a defensive reaction after the town was laid waste by the Danish King Sweyn in 1003. It is just sheer luck that they have been frozen in time by later additions.