Holland is a historical region in the western part of the Netherlands.
Part of Arthur’s empire according to the Alliterative Morte Arthur, Holland did not exist as a country until the tenth century.
Holland | 0 to 700 AD
During the early centuries of the Common Era, the Roman Empire extended its influence into the Low Countries, including the area that would later become Holland. The Romans established several forts and settlements in the region, and they introduced Roman infrastructure and culture. The town of Voorburg (Forum Hadriani) was one of the Roman settlements in what is now South Holland. The Romans referred to the general region that encompasses modern-day Holland as Batavia or Germania Inferior.
With the decline and eventual collapse of the Western Roman Empire in the late fifth century, the Low Countries experienced the effects of the Migration Period. Various Germanic tribes, including the Frisians, Saxons, and Batavians, inhabited the region and played a role in shaping its early medieval history. The Frisians, in particular, were known to inhabit parts of what is now North Holland.
The introduction of Christianity to the Low Countries began from the fourth century. Christian missionaries, including Saint Willibrord, played a significant role in converting the local populations. Churches and monasteries were established, contributing to the spread of Christianity in the region.
Towards the end of the eighth century, Viking raids and incursions became a significant threat to the coastal regions of what would later become Holland. These raids led to the construction of fortifications and defensive structures along riverbanks and coastlines.
By the seventh century, local dynasties and rulers began to emerge in the Low Countries. The Frisians had their own kings and rulers, and the region witnessed the development of small-scale feudal structures.
Source
Alliterative Morte Arthure | c. 1400