The First Fortifications
The first fortification in the area was Portchester Castle which ultimately dates back to the Roman Portus Adurni. Its position allowed it to provide a degree of defence for the upper reaches of Portsmouth Harbour. The first recorded fortifications on Portsea island appear to have been a wall constructed in 1212 around a primitive shipyard sited just to the north of the camber. The wall lasted less than twelve years.
Portsmouth was subject to attacks in 1338, 1369, 1377 and 1380. It seems likely that after these attacks a simple earthwork was constructed around the then town. The earthwork aside the first defensive structure built to defend Portsmouth proper was the then wooden Round Tower built between 1418 and 1426 as part of a series of works around the town. A second wooden tower was built on the opposite side of the harbour 4 years later.
Read more about this topic: Fortifications Of Portsmouth