Tintagel is something of an anomaly on many counts. It was never a militarily important location, and rarely was it a royal residence. It is seen as key part of Arthurian myth, but there is little suggestion King Arthur ever set foot there. It has been inhabited since Roman times, but the only fortifications which remain are harking back to something that never happened.
The great King Arthur was a man, and thus needed to be born and grow up. Before even that he needed to be conceived, and it is at Tintagel that that night occurred, at least according to Geoffrey of Monmouth. Uther Pendragon, Arthur’s father-to-be and King of All Britons fell in love with Ygrayne when he saw her at his coronation. But her husband Gorlois, the Duke of Cornwall was not happy about that and left, taking her with him. Uther used that as an excuse to go to war with Gorlois. There is no suggestion of Ygrayne’s position on the matter – the least worst option here is that she was receptive to Uther’s advances, which is the position Mary Stewart takes.
Gorlois led his campaign from a castle called Dimilioc, but he sent his wife to Tintagel as its location made it impossible to breach – Uther is told that three armed warriors could defend the castle against the entirety of Britain – why Gorlois didn’t choose to base himself out of Tintagel as well I don’t know! Regardless, Uther believed this claim, and so called for Merlin’s aid. He asked Merlin to magically disguise him as Gorlois so he could slip past the defences. Merlin worked his magic and created a potion to do the job.