According to the Alliterative Morte Arthure, Pisa was acquired by Arthur after the Roman War.
Pisa | 0 to 800 AD
Pisa, like many other cities in Italy, has ancient origins dating back to the Etruscan and Roman periods. During the Roman Republic and Empire, Pisa likely functioned as a port and trading center, given its proximity to the Tyrrhenian Sea. The Roman presence in the region contributed to the development of infrastructure, trade routes, and urbanization.
Early Medieval Period | 5th – 8th centuries AD
With the decline of the Western Roman Empire in the fifth century, Italy experienced significant political and social changes. Barbarian invasions, including those by Visigoths and Ostrogoths, and Vandals impacted the region. The Lombards, a Germanic people, invaded Italy in the late sixth century and established their rule. It’s likely that Pisa, along with other cities, came under Lombard control during this period.
In some instances, certain coastal areas in Italy, including Pisa, came under Byzantine influence as the Byzantine Empire sought to maintain control over parts of the Italian Peninsula. However, the Lombards eventually asserted their dominance over much of northern and central Italy, and Pisa likely fell under Lombard rule.
Alliterative Morte Arthure | c. 1400