With regards to Welsh history and written records, there is a long trail of suppression, first by the Normans, then by the "English".
The Chronicle of the Princes (Brut y Tywysogyon) states a colony was established in 1105 when Henry I allowed a number of Flemings from modern-day Belgium to settle in the area around Haverfordwest. They were later joined by English settlers - the Flemish and English languages were similar at the time. This led to the extinction of Welsh in the area, and a legacy of aggression towards the language which has only softened in recent times.
The Conquest of Wales was a traumatic event. When Llywelyn ap Gruffudd was killed in 1282 in a skirmish with the Anglo-Norman army at Cilmeri, near Builth Wells, one chronicler wrote that the whole of Wales was thrown to the ground. The independent princes of Wales had been the main patrons of the poets known as Y Gogynfeirdd and Llywelyn's death caused an anguished response. After almost a thousand years, the political independence of the Welsh came to an end and the culture and customs of the country faced annihilation.
Richard II attempted to prohibit writing in Welsh c.1390, to prevent communication of Welsh ideas and propaganda. Next, Henry IV, had to deal with the rebellion of Owain Glyndwr.
Act of Union of 1536. Passed by Henry VIII, the act meant Wales was legally incorporated into England. The act also contained the infamous language clause, which banned Welsh monoglot speakers from public office.
By the time Caxton brought the printing press to England, Richard III was on the throne, and c.1483 he banned the use of the printing press outside London. A bit like trying to ban subversive websites today. Onward to Henry VIII, in 1542, the Laws in Wales Act was passed that banned the use of Welsh in legal proceedings. Oliver Cromwell contributed by persecuting the established Welsh Church. By the time of the German Hanovian kings, historians became more pro-Saxon, and anti-Welsh. The same happened again after Prince Albert married Queen Victoria. One of the final straws c.1846 were language and education "reforms" that forbade the teaching of Welsh in Welsh schools. Welsh-speaking teachers were replaced with English ones, and speaking Welsh was punished.
The thing is, we all know it's a minefield of conflicting stories and legends, and we have to tread carefully. The elephant in the corner is Geoffrey of Monmouth. What did Geoffrey do so wrong? He attracts a lot of derogatory comments from modern historians. Differences of opinion between historians is one thing, but our modern historians go one big step further and declared that his histories as nothing but myth, legend or worse. The orthodox historians piss on anything to do with Geoffrey of Monmouth, even though the poor guy was just (it seems) translating from Welsh to Latin.
In other topics and posts we have already covered this quite well, in so much as the Normans, Tudor, Georgian and Victorian historians all had their own historians with their own agendas, and distorted a lot of what had gone before. So I won't rehash that, except to note that their justification for rubbishing Geoffrey's accounts are largely and originally based on William of Malmsbury who accused Geoffrey of fraud.
This is actually a strange situation. William was not an unbiased observer. Why? After Geoffrey died, William has applied for Geoffrey's job as Bishop of Llandaff, but had been rejected, and was vehemently anti-Welsh from that point onwards. When Athelstan had expelled the Welsh from Devon and Cornwall, William had called them "polluted vermin".
When Jeffrey ap [son of] Arthur, Lord Bishop of Llandaff [Geoffrey Monmouth], died, an Englishman of the name Gwilym Bach [little William or William the Less] arrived, of whom I have already spoke, who desired Dafydd ap Owen, Prince of Gwynedd, to make him Bishop in Geoffrey’s place about the year 1169 AD. But when it was not in the mind of Dafydd ap Owen to grant him his request the man went home full of hatred and commenced to exercise his mind how best to despise and malign not only the memory of this bishop, who was lying in his grave, but also the whole of the Welsh nation.
Ref = Mirror of the Principal Ages by Rev.Theophilus Evans.
Despite that, our modern historians still run with the idea that Geoffrey's history is myth and legend. Regardless of the portions of history that match with other "recorded" history. For example, his account of the Romans in Britain match fairly well with other accounts, and even the Romans' own account despite their well-known tendency for "bigging-up" everything they did at the expense of everyone else's reputation. (See Julius Caesar's accounts of the Gaul Wars)
Geoffrey's account of Julius Caesar's invasion of Britain in 55BC fairly resembles what we know now from other sources. The Roman's campaign got off to a good start, managing to cross the Channel. But it ground to a halt after crossing the Thames and Caesar was forced to make peace with Cassivaelaunus, before withdrawing. The Romans (of course) did not call it a retreat. Similarly with Claudius' invasion c.100 years later, Claudius ended the war not by conquest but by marrying his own daughter Genvissa to Arviragus, King of Siluria, in 45 AD. Siluria was a kingdom in the south of Wales. Their son was Meric (Marius), King of the Britons. It was only then after that the Romans, with the aid of Southern Britains, proceeded north.
Modern English translation of the above passage, as given in Geoffey of Monmouth, The History of the Kings of Britain, trans. Lewis Thorpe (London: 1966), p. 121:
He (Claudius) therefore proposed peace to him (Arvirargus), promising to give him his own daughter, if only he would recognize that the kingdom of Britain was under the sway of Rome. His nobles persuaded Arvirargus to abandon his plans for battle and to accept the proposals of Claudius. Their argument was that it could be no disgrace for him to submit to the Romans, since they were the acknowledged overlords of the whole world. Arvirargus was swayed by these arguments and by others of a similar nature. He accepted their advice and submitted to Claudius. Claudius soon sent to Rome for his daughter. With the help of Arvirargus he subdued the Orkneys and the other islands in that neighbourhood. http://www.deloriahurst.com/deloriahurst%20page/1844.html
If we still don't like Geoffrey, we surely should think we could rely on Roman/Latin accounts instead. Not so! Here's just one example. Vespasian matters in British history (and Mithraism/proto-Christianity, but that's another story). According to Tacitus (ii.97), his rule was "infamous and odious" but according to Suetonius (Vesp. 4), he was "upright and, highly honourable".
Confused? We should be.
The stuff about Carausius, as a British Roman Emperor, seems to support that.
.. his appointment to command the Classis Britannica, a fleet based in the English Channel, with the responsibility of eliminating Frankish and Saxon pirates who had been raiding the coasts of Armorica and Belgica. https://en.wikipedia.org/wiki/Carausius
This was c.286 - so the sneaky Saxons were bothering the Brits for centuries before Vortigen. Some might say the arrival of the Romans just delayed that. Others might say it wos the Roman's wot dunnit, by weakening the Brits so much that the Saxons reached a critical mass.
Carausius is reported to be buried in St Tudclud's church in Penmachno, Conwy. https://thejournalofantiquities.com/tag/inscribed-stones-at-penmachno-in-wales/