St Michaels Mount is now owned by the National Trust. The island that St Michaels Mount sits on is connected to the mainland by a causeway that appears at low tide. During its long history it has been a priory, fortress, a place of pilgrimage, the home of the St Aubyn family in 1659, and now open to the public by the National Trust.
You can walk through the castle rooms, the armoury, a rococo Gothick drawing room and, at the highest point, a 14th-century church. The somewhat steep and rugged walk up to the castle gives views all round. Apart from the castle, the island has 25 residents in its village, a church, harbour and beautiful gardens. The National Trust has a gift shops, restaurant and an audio-visual presentation on the history of the castle.
The causeway is exposed at low tide allowing you to wander over by foot, otherwise there are small boats that run to and from the mainland. With this and the steep climb it is advisable to wear sturdy footwear.
St Michaels Mount according to legend was built by a giant called Cormoran. The greenstone on the island is supposed to have been brought there by the Giantess Cormelian, wife of the Giant Cormoran, in her apron. Cormoran, building the mount of granite, saw that she had brought the wrong rock, and killed her. Cormelian dropped the stones as she fell and is said to be buried under the pile of greenstone you can still find there today.
St Michaels Mount was a trading post for tin and copper from as early as 350 BC. But little is recorded of what happened here, until a visit by St. Keyne who visited the rock in 490 AD. The story goes that the archangel St Michael appeared to a group of fishermen at the mount in 495. The archangel was a busy man at this time, and was said to have appeared in some 400 places throughout Britain and Europe during the 4th and 5th centuries.
Edward the Confessor granted the mount to the Benedictine monks from Mont St Michel in France, and by 1135 they had built a priory on the summit of the mount. They also constructed a harbour and causeway.
Henry V granted it to the Abbey of Syon in Twickenham. Then of course Henry VIII grabbed it when he dissolved the monasteries.
It changed then from a church to a fortress, and was a Royalist stronghold during the Civil War. The commander of the Parliamentarian forces that captured the mount during the Civil War, one Colonel John St Aubyn, bought it from the state, and one of his descendants, Lord St Levan, still lives there.
Lord St Levan gave St Michaels Mount to the National Trust in 1954, but still lives there. Today around half a million visitors a year visit the site. It can be reached by causeway at low tide, or from a passenger ferry at high water.
St Michaels Mount, Cornwall genealogical information from Genuki