It was one of only three permanent legionary fortresses in later Roman Britain and, unlike the other sites at Chester and York, its archaeological remains lie relatively undisturbed beneath fields and the town of Caerleon and provide a unique opportunity to study the Roman legions in Britain. Excavations continue to unearth new discoveries; in the late 20th century a complex of very large monumental buildings outside the fortress between the River Usk and the amphitheatre was uncovered. This new area of the "canabae" was previously unknown. The Brythonic name "Isca" means "water" and refers to the River Usk. The suffix "Augusta" appears in the Ravenna Cosmography and was an honorific title taken from the legion stationed there. The place is commonly referred to as "Isca Silurum" to differentiate it from Isca Dumnoniorum and because it lay in the territory of the Silures tribe. However, there is no evidence that this form was used in Roman times. The later name, "Caerleon", is derived from the Welsh for "fortress of the legion". Isca was founded in 74 or 75 during the final campaigns by Governor Sextus Julius Frontinus against the fierce native tribes of western Britain, notably the Silures in South Wales who had resisted the Romans’ advance for over a generation. Isca became the headquarters of the Legion II Augusta based in the large fortress of typical legionary "playing-card" shape and built initially with an earth bank and timber palisade. It remained their headquarters until at least 300 AD. The interior was fitted out with the usual array of military buildings: a headquarters building, legate's residence, tribunes' houses, hospital, large bath house, workshops, barrack blocks, granaries and, unusually, a large amphitheatre.. At this time there were four legions in Britain out of a total of about 30 legions in the Empire, making Britain one of the most heavily militarised provinces due to its frontier status and hostile neighbours. Each legion consisted of over 5,000 heavily armed and highly disciplined professional soldiers who enlisted in the army for at least 20 years. As the backbone of the army, legionaries were the conquerors and builders of the Roman Empire who brought with them foreign ideas, practices and traditions that would change the society and culture of Britain forever. An inscription of Trajan gives a date of AD99/100 for the replacement of the fortress walls, when the original earth and timber ramparts of the fortress were strengthened by the addition of a stone revetment at the front.