The first writer to connect Arthur with Tintagel was GEOFFREY OF MONMOUTH:
ARTHURIAN PASSAGES FROM THE HISTORY OF THE KINGS OF BRITAIN

CHAP. XIX.--Uther, falling in love with Igerna, enjoys her by the assistance of Merlin's magical operations.

After this victory Uther repaired to the city of Alclud, where he settled the affairs of that province, and restored peace everywhere. He also made a progress round all the countries of the Scots, and tamed the fierceness of that rebellious people, by such a strict administration of justice, as none of his predecessors had exercised before: so that in his time offenders were everywhere under great terror, since they were sure of being punished without mercy. At last, when he had established peace in the northern provinces, he went to London, and commanded Octa and Eosa to be kept in prison there. The Easter following he ordered all the nobility of the kingdom to meet at that city, in order to celebrate that great festival; in honour of which he designed to wear his crown. The summons was everywhere obeyed, and there was a great concourse from all cities to celebrate the day. So the king observed the festival with great solemnity, as he had designed, and very joyfully entertained his nobility, of whom there was a very great muster, with their wives and daughters, suitably to the magnificence of the banquet prepared for them. And having been received with joy by the king, they also expressed the same in their deportment before him. Among the rest was present Gorlois, duke of Cornwall, with his wife Igerna, the greatest beauty in all Britain. No sooner had the king cast his eyes upon her among the rest of the ladies, than he fell passionately in love with her, and little regarding the rest, made her the subject of all his thoughts. She was the only lady that he continually served with fresh dishes, and to whom he sent golden cups by his confidants; on her he bestowed all his smiles, and to her he addressed all his discourse. The husband, discovering this, fell into a great rage, and retired from the court without taking leave: nor was there any body that could stop him, while he was under fear of losing the chief object of his delight. Uther, therefore, in great wrath commanded him to return back to court, to make him satisfaction for this affront. But Gorlois refused to obey; upon which the king was highly incensed, and swore he would destroy his country, if he did not speedily compound for his offence. Accordingly, without delay, while their anger was hot against each other, the king got together a great army, and marched into Cornwall, the cities and towns whereof he set on fire. But Gorlois durst not engage with him, on account of the inferiority of his numbers; and thought it a wiser course to fortify his towns, till he could get succour from Ireland. And as he was under more concern for his wife than himself, he put her into the town of Tintagel, upon the sea-shore, which he looked upon as a place of great safety. But he himself entered the castle of Dimilioc, to prevent their being both at once involved in the same danger, if any should happen. The king, informed of this, went to the town where Gorlois was, which he besieged, and shut up all the avenues to it. A whole week was now past, when, retaining in mind his love to Igerna, he said to one of his confidants, named Ulfin de Ricaradoch: "My passion for Igerna is such, that I can neither have ease of mind, nor health of body, till I obtain her: and if you cannot assist me with your advice how to accomplish my desire, the inward torments I endure will kill me."-- "Who can advise you in this matter," said Ulfin, "when no force will enable us to have access to her in the town of Tintagel? For it is situated upon the sea, and on every side surrounded by it; and there is but one entrance into it, and that through a straight rock, which three men shall be able to defend against the whole power of the kingdom. Notwithstanding, if the prophet Merlin would in earnest set about this attempt, I am of opinion, you might with his advice obtain your wishes." The king readily believed what he was so well inclined to, and ordered Merlin, who was also come to the seige, to be called. Merlin, therefore, being introduced into the king's presence, was commanded to give his advice, how the king might accomplish his desire with respect to Igerna. And he, finding the great anguish of the king, was moved by such excessive love, and said, "To accomplish your desire, you must make use of such arts as have not been heard of in your time. I know how, by the force of my medicines, to give you the exact likeness of Gorlois, so that in all respects you shall seem to be no other than himself. If you will therefore obey my prescriptions, I will metamorphose you into the true semblance of Gorlois, and Ulfin into Jordan of Tintagel, his familiar friend; and I myself, being transformed into another shape, will make the third in the adventure; and in this disguise you may go safely to the town where Igerna is, and have admittance to her." The king complied with the proposal, and acted with great caution in this affair; and when he had committed the care of the siege to his intimate friends, underwent the medical applications of Merlin, by whom he was transformed into the likeness of Gorlois; as was Ulfin also into Jordan, and Merlin himself into Bricel; so that nobody could see any remains now of their former likeness. They then set forward on their way to Tintagel, at which they arrived in the evening twilight, and forthwith signified to the porter, that the consul was come; upon which the gates were opened, and the men let in. For what room could there be for suspicion, when Gorlois himself seemed to be there present? The king therefore stayed that night with Igerna, and had the full enjoyment of her, for she was deceived with the false disguise which he had put on, and the artful and amorous discourses wherewhith he entertained her. He told her he had left his own place besieged, purely to provide for the safety of her dear self, and the town she was in; so that believing all that he said, she refused him nothing which he desired. The same night therefore she conceived of the most renowned Arthur, whose heroic and wonderful actions have justly rendered his name famous to posterity

Translated by J.A. Giles

DOUGLAS B. W. SLADEN
AUGUST 1884.

TINTAGEL, huge rock-royal, glad was I
    That only here and there a crumbling wall,
    Hard to distinguish from the natural,
Still stood upon thy summit. Worthily
Could feudal palace-keep scarce occupy
    Such site; and how would newer buildings pall
    Where every rood was stamped historical,
Or fancy-tinged, or steeped in legendry?
    Dismantled, one can picture on the isle
       A shadowy Arthur washed up from the bay,
    And rear upon its front a stately pile
       Of marble as kings reared them in the day,
    Ere time had taught the Briton to neglect
    The lesson of the Roman Architect.

  "Sir, there be many rumours on this head:
For there be those who hate him in their hearts,
Call him baseborn, and since his ways are sweet,
And theirs are bestial, hold him less than man:
And there be those who deem him more than man,
And dream he dropt from heaven: but my belief
In all this matter--so ye care to learn--
Sir, for ye know that in King Uther's time
The prince and warrior Gorlois, he that held
Tintagil castle by the Cornish sea,
Was wedded with a winsome wife, Ygerne:
And daughters had she borne him,--one whereof,
Lot's wife, the Queen of Orkney, Bellicent,
Hath ever like a loyal sister cleaved
To Arthur,--but a son she had not borne.
And Uther cast upon her eyes of love:
But she, a stainless wife to Gorloïs,
So loathed the bright dishonour of his love,
That Gorloïs and King Uther went to war:
And overthrown was Gorloïs and slain.
Then Uther in his wrath and heat besieged
Ygerne within Tintagil, where her men,
Seeing the mighty swarm about their walls,
Left her and fled, and Uther entered in,
And there was none to call to but himself.
So, compassed by the power of the King,
Enforced she was to wed him in her tears,
And with a shameful swiftness: afterward,
Not many moons, King Uther died himself,
Moaning and wailing for an heir to rule
After him, lest the realm should go to wrack.
And that same night, the night of the new year,
By reason of the bitterness and grief
That vext his mother, all before his time
Was Arthur born, and all as soon as born
Delivered at a secret postern-gate
To Merlin, to be holden far apart
Until his hour should come; because the lords
Of that fierce day were as the lords of this,
Wild beasts, and surely would have torn the child
Piecemeal among them, had they known; for each
But sought to rule for his own self and hand,
And many hated Uther for the sake
Of Gorloïs. Wherefore Merlin took the child,
And gave him to Sir Anton, an old knight
And ancient friend of Uther; and his wife
Nursed the young prince, and reared him with her own;
And no man knew. And ever since the lords
Have foughten like wild beasts among themselves,
So that the realm has gone to wrack: but now,
This year, when Merlin (for his hour had come)
Brought Arthur forth, and set him in the hall,
Proclaiming, "Here is Uther's heir, your king,"
A hundred voices cried, "Away with him!
No king of ours! a son of Gorloïs he,
Or else the child of Anton, and no king,
Or else baseborn." Yet Merlin through his craft,
And while the people clamoured for a king,
Had Arthur crowned; but after, the great lords
Banded, and so brake out in open war

Alfred Tennyson: The Idylls of the King