I would like to know if the medieval people in Britain, by around 800 AD, knew that there was an empire that ruled Britain several hundreds of years before.
Otherwise, were there any myths, legends, etc related to Hadrian's Wall, Roman ruins, writings, or any significant trace of Romans in folklore?
Educated Britons would've received an education deeply steeped in classical antiquity, so knowledge of the Roman Empire must have been inevitable among the literate. Unsurprisingly, therefore, many writings from this time period mention Roman Britain. Notable examples include the Anglo-Saxon Chronicle and the Historia ecclesiastica gentis Anglorum, both of which mention Rome. These date to around the 9-12th centuries and the 8th century, respectively.
The common folk would have been acquainted with the Romans too, both because of knowledge filtering through from the learned class, and because of retained traditions. Much of these folklore were later recorded by scribes or poets, or otherwise served as inspiration for their literary traditions. Examples include pseudohistorical works that speak of Rome, such as the fantastical 9th-century Historia Brittonum and the 12th-century Historia Regum Britanniae.
The latter became especially popular during the High Middle Age and gave rise to a lot of British folklore. Among them is the legend of Maximianus, "the only Roman emperor to occupy an important place in later British folklore[1]." This is the same story @ForestPines mentioned earlier, and probably predates Geoffrey of Monmouth writing it down in his Historia.
Thus, we may safely conclude Medieval Britons indeed knew about the Romans.
[1]: Jones, Michael E. The End of Roman Britain. Cornell University Press, 1998.
Educated people in the European Middle Ages knew Latin and read the Roman classics. They were thus very well informed about the Roman Empire. Even uneducated people were keenly aware of the contents of the Bible (through sermons, passion plays, for example). The Roman Empire figures very prominently in the New Testament narrative (Caesar Augustus, Pontius Pilate, Give unto Caesar… etc. etc.).
The short answer: they were very aware of the Roman empire and its past glory.
Long answer:
This questions gets to the problem of "What was England Like in the 9th Century?" If you can answer this question well, you can get an idea of whether the average person would known of Rome, or how much they would have known. I would suggest as a starter looking at a simple chronology like England in the 9th Century. As you can see there were a lot of small kingdoms and lots of wars and invasions. Virtually no literary or historical works are known from the period. The Saxon Chronicle was begun during this period, but not published in any way.
Nevertheless, there are some works, the most extensive being those of the poet Cynewulf. His longest poem, which is actually an adventure story, is Elene, which describes the great battle between the Goths/Huns and the Romans/Franks. The hero of this poem is Constantine, the king of the Romans. Of course, nowadays, we consider Constantine of Britain to be a false emperor, and know that he died before the Huns reached the lands of the Franks. Nevertheless, to Cynewulf and his audience Constantine was the great Emperor of Rome and its people resident in England and Gaul who fought the barbarian Huns and saved England. Here is a translated extract from the poem:
But the king was fear-smitten, awed with terror, as he looked upon the hostile host, the army of the Huns and Goths, that upon the river's bank at the boundary of the Roman realm was massing its strength, an uncounted multitude. The king of the Romans suffered bitter grief of soul, and hoped not for his kingdom because of his small host; he had too few warriors, trusty thanes, to encounter the overmight of brave men in battle.
Later the poem describes how King Constantine of the Romans was converted to Christianity and speaks of Eusebius, "bishop of Rome".
There were actually many Roman cities in England and even in the 8th century "Rome" still existed in England in particular communities containing real colonists descended from actual Romans. So, not only were the people in England aware of Rome, they considered them to be friends and neighbors, even. There is a very interesting book, recently published, which is a micro-study of one such town, called Wasperton. The book is called "Wasperton: A Roman, British and Anglo-Saxon Community in Central England.". As you can read in this book, Latin was still widely being spoken in England in the 8th century among ordinary people. Many people even considered themselves to be actual "Romans" even though they were "temporarily" being ruled by barbarian kings.
Now, you might ask, how could someone consider themself to be a Roman, when their king is a Saxon? The answer is that often the Saxon king was far away and the real leader of the community is a bishop, as was often the case. The bishop speaks Latin, every Sunday you go to mass and hear the Latin, you and your friends speak Latin and British. The king might be Saxon, but he speaks a strange language and is not even a Christian and he lives in a town far away. You and your friends live a Roman life, even though Rome itself has been destroyed and the capital moved to Ravenna.
Latin was the lingua franca of the entire island and it was widely spoken by not just ex-Roman communities like Wasperton, Withington and Viroconium, but in many other areas. This is what Bede wrote in the first sentence of his book on the history of Britain:
At the present time, there are five languages in Britain, just as the divine law is written in five books, all devoted to seeking out and setting forth one and the same kind of wisdom, namely the knowledge of sublime truth and of true sublimity. These are the English, British, Irish, Pictish, as well as the Latin languages; through the study of the scriptures, Latin is in general use among them all.
Also, the first line from the Saxon Chronicles:
The island Britain is 800 miles long, and 200 miles broad. And there are in the island five nations; English, Welsh (or British), Scottish, Pictish, and Latin.
Thus, not only did everyone in England know of the glory of Rome past, many lived it as fallen and conquered Romans hoping for a restoration.
The early medieval Welsh had several folktales and legends which survive in versions written down in the 12th century, but which refer to the Romans. The best known example is probably the Dream Of Emperor Macsen, whose title character is derived from Magnus Maximum, commander of the Roman army in Britain in the late 4th century.
To answer the question about Hadrian's Wall directly, Gildas mentions the conquest and loss of Britain to the Romans, including the construction of the wall, in De Excidio et Conquestu Britanniae ("On the Ruin and Conquest of Britain"). I am not sure exactly when it was written, but Gildas dies in roughly 570.
Gildas was British and spent a great deal of his life in Britain, so I think that is good evidence for educated knowledge of the Roman Empire, its influence and effects, in Britain, but Gildas retired to Brittany, and I am not sure if he wrote De Excidio there.
Gildas's account of the wall's construction is rather confused. He appears to describe the construction of Hadrian's Wall as an act of the Roman occupiers very late in the period of their occupation (as late as Honorius perhaps?). Rather than in Hadrian's time.
@Sempahore has mentioned Bede, and as it happens Bede also mentions the wall in Historia ecclesiastica gentis Anglorum (writing in 731), although he ascribes it to Septimius Severus:
Having been victorious in all the grievous civil wars which happened in his time, he was drawn into Britain by the revolt of almost all the confederate tribes; and, after many great and dangerous battles, he thought fit to divide that part of the island, which he had recovered from the other unconquered nations, not with a wall, as some imagine, but with a rampart. For a wall is made of stones, but a rampart, with which camps are fortified to repel the assaults of enemies, is made of sods, cut out of the earth, and raised above the ground all round like a wall, having in front of it the ditch whence the sods were taken, and strong stakes of wood fixed upon its top. Thus Severus drew a great ditch and strong rampart, fortified with several towers, from sea to sea;
From which it is clear that educated Britains did have some understanding of the history of Roman Britain in the 8th century. As to what "normal" or "average" people knew - I doubt that is something we can ever determine precisely at this late date.
Discovering the opinions of uneducated, non-writing, people in times before opinion polls is inherently difficult, regardless of the topic. The best approach I can see to this question is to look at what the Church was teaching, and assume it was generally believed.
For this purpose the cult of St. Alban is interesting. According to the website of the Cathedral and Abbey Church of St. Alban, "Alban was probably buried in the Roman cemetery to the south of the present Abbey Church. Recent finds suggest an early basilica over the spot and later a Saxon Benedictine monastery was founded, probably by King Offa around 793."
If there were a sufficiently active cult to inspire founding of a monastery, presumably St. Alban's story would have been the subject of church teaching, at least locally, making it accessible to ordinary people. Roman rule over Britain is an inherent part of that story.
According to History World, the Brits interacted enough with the Holy Roman Empire on the continent to participate in their preservation of classic scrolls. Charlemagne certainly knew of the then-current Byzantine Empire, the Moorish kingdoms, and the history of Rome. The word surely reached the occasional educated British ear. This source does not refer to the standards of British world-knowledge of the time, or to the Celtic scholarship that was going on, apparently in isolation from other educated communities.
Alcuin, a distinguished teacher from York, is invited in 780 to found a school in the palace at Aachen (Charlemagne and his family sometimes join the lessons); and the copying of manuscripts is carried out in a beautiful script which later becomes the basis of Roman type. Though still primitive by the standards of classical culture, the renewal of intellectual and artistic life under Charlemagne has justly been described as the Carolingian Renaissance.
It would be quite difficult to not to know about Roman Empire in medieval Europe just because the events described in New Testament and other religious literature happened in Roman Empire. This includes for instance, the execution of Jesus as well as deeds and life of various saints.
The proper question would be how did the medieval population imagine Roman Empire. There were no images and even in many books we can see biblical events depicted as if they happened in medieval Europe (in regards of clothing, architecture etc).
So, in short, they knew that there was such empire, but could know very little about what a state it was.
The Catholic Church had a presence in Britain from 3rd Century (see St Alban who was a Roman soldier and martyr after whom the city of St Albans was named). The Church continued and grew through the centuries until now, but in medieval times the community life of the country was extremely orientated around the Church's liturgical characters. This link with Rome and this history of the Church and the high percentage of Britains who were practicing Catholics (amazing numbers about how many were monks and priests in society) and the tradition in the Catholic Church of teaching about martyrdoms - especially local martyrs - suggests that they did know.
I want to expand a point raised in @Semaphore's excellent answer. Bede's Ecclesiastical History of the English People not only mentions Rome - it gives a detailed (if garbled at times) account of Roman history, from the British perspective. Here's the beginning of the list of contents of Book I: (source)
BOOK I
I. Of the situation of Britain and Ireland, and of their ancient inhabitants
II. Caius Julius Caesar, the first Roman that came into Britain
III. Claudius, the second of the Romans who came into Britain, brought the Islands Orcades into subjection to the Roman Empire; and Vespian, sent by him, reduced the Isle of Wight under their dominion
IV. Lucius, king of Britain, writing to Pope Eleutherus, desires to be made a Christian
V. How the Emperor Severus divided that part of Britain, which he subdued, from the rest by a rampart
VI. The reign of Diocletian, and how he persecuted the Christians
VII. The passion of St. Alban and his companions, who at that time shed their blood for our Lord [A.D. 305]
VIII. The persecution ceasing, the church in Britain enjoys peace till the time of the Arian heresy [A.D. 307337]
IX. How during the reign of Gratian, Maximus, being created emperor in Britain, returned into Gaul with a mighty army [A.D. 383]
X. How, in the reign of Arcadius, Pelagius, a Briton, insolently impugned the grace of God
XI. How, during the reign of Honorus, Gratian and Constantine were created tyrants in Britain; and soon after the former was slain in Britain, and the latter in Gaul
XII. The Britons, being ravaged by the Scots and Picts, sought succor from the Romans, who, coming a second time, built a wall across the island; but the Britons being again invaded by the aforesaid enemies, were reduced to greater distress than before
XIII. In the reign of Theodosius the younger, Palladius was sent to the Scots that believed in Christ; the Bretons begging assistance of Ætius, the consul, could not obtain it [A.D. 446]
XIV. The Britons, compelled by famine, drove the barbarians out of their territories; soon after there ensued plenty of corn, luxury, plague, and the subversion of the nation [A.D. 426–447]
XV. The Angles, being invited into Britain, at first obliged the enemy to retire to a distance; but not long after, joining in league with them, turned their weapons upon their confederates [A.D. 450–456]
XVI. The Bretons obtained their first victory over the Angles, under the command of Ambrosius, a Roman
I think this really clinches it.
This is not a proper answer, but more of an extended comment on other answers.
I note the answers and possibly the question tend to not clearly distinguish England from Britain. [This is a common problem with sources in English.]
'Middle ages' is a broad period, so specifically calling out 800AD is helpful.
At that time, Britain was populated by Scots, Picts, Anglo-Saxons and Welsh.
All of these groups were Christian by 800AD; others have noted the implications for clerical and aristocratic knowledge.
The Scots and Picts territory never was Romanised, so their view ought be to be different to the Anglo-Saxons.
The Welsh were the descendents of the Roman Britons. This means that both the secular and clerical communities had continuity with Roman Britain. Wikipedia suggests that "there is ongoing debate as to the extent of a lasting Roman influence being applicable to the early Middle Ages in Wales". My only point is that popular knowledge might well have been quite different to that of the English.
I've read something very specific on the subject in "Britain after Rome" by Robin Fleming. According to this book that was very subjective depending on your social status. Bear in mind at this time that the artefacts and art of Rome was everywhere in Britain and plain to see. The Saxon ruling class often inhabited Roman-built villas, and the only stone buildings that existed were from the time Rome (Saxons had lost the art of stone masonry), complete with the art and architecture. Much more plentiful than today as the majority of Roman buildings were plundered for stone between the 12th and 18th centuries. And so it was definitely a hot topic to the new inhabitants. The viewpoint can be divided into two: The ruling class and clergy were acutely aware of the Empire. Many had been to the continent and Rome itself, where knowledge of the empire was a matter of fact, and where the traditions and culture still existed in diminished form. However most layman Saxons accepted an interpretation that people who built the environment around them were an extinct race of giants. This slightly mythical viewpoint lasted until the later medieval period, when events such as the Norman conquest and participation in the crusades made knowledge of Rome mainstream again, as it is today.
