A series of Roman forts more or less occupied the same site in Cardiff, on a river terrace adjacent to the River Taff, and represent continuous occupation from the late 50s AD until the late fourth century.
The main, outer walls of Cardiff Castle are largely a late nineteenth-early twentieth century rebuilding of a late Roman fort enclosure. This comprised a stout wall enclosing a near square area of about 3.4ha, punctuated by fourteen regularly spaced polygonal towers and two twin towered gateways. This last fort was built from the mid third century over an extensive open settlement. This appears to have extended at least 200m south of the castle into the area of the modern town. The fort walls blocked the settlement's main north-south road. At some point it was involved in extensive iron production and working, resulting in great drifts of slag. Its core area, roughly correspoding to the later fort, may have been enclosed by a rampart and ditch.
This settlement replaced earlier phases of military settlement which centred on what was probably an auxiliary fort enclosure, whose southern rampart lay within the northern part of the present castle enclosure. The fort was established in the 70s AD and its rampart was refurbished around 100AD. Timber-framed buildings flanked the road running from its south gate, extending at least as far as the later settlement.
The military settlement overlay an earlier settlement, established around 50AD and including several large timber framed buildings, one at least 45m by 25m. It is not known how far this settlement extended or whether it was enclosed. It appears to have been founded in the aftermath of the defeat of the Roman army by the Silures, the native tribe of the region, in the same phase as the foundation of the legionary fortress at Usk.
The fort walls were comprehensively examined as they were rebuilt from 1889 onwards. Excavations have been conducted within the castle enclosure from 1974 to 1984, in 2003 and in 2005-6 (for which see Glamorgan-Gwent Archaeological Trust website).
The later castle is recorded under NPRN 33.
Sources: Ward in Archaeologia vol. 57.2 (1901), 335-52
in Archaeologia Cambrensis 6th series 8 (1908), 29-64
68 (1913), 159-64
69 (1914), 407-16
Wheeler in the Antiquaries Journal 2 (1922), 361-70
Jarrett in 'The Roman Frontier in Wales' 2nd edition (1969), 70-3
RCAHMW 1976 Glamorgan Inventory I.2 - Iron Age & Roman - (1976), 90-4
Webster in Morgannwg 25 (1981), 201-11
in Trivium 25 'Conquest, Co-existence & Change' (1991), 35-9
Evans in Archaeology in Wales 44 (2004), 43-60
John Wiles, RCAHMW, 30 August 2007