The military historian Sir Charles Oman said of the castle's situation "Trematon is high aloft, on one of the summits of the rather chaotic group of hill-tops which lie behind Saltash and its daring modern bridge.

James McKenzie writes that an earthwork fortress on the site belonged to the Saxon earls of Cornwall, while historian William Woolwater calls it an "ancient palace of the Cornish Kings", built before 959.

[3] In 1542, antiquary John Leland wrote: "The greaunt and auncient Castelle of Tremertoun is upon a Rokky Hille; wherof great peaces yet stond, and especially the Dungeon.

"[3] When Sir Francis Drake returned from his circumnavigation voyage in 1580, he came into harbour in Plymouth, then slipped out to anchor behind St Nicholas Island until word came from Queen Elizabeth's Court for the treasures he had gathered to be stored in Trematon Castle.

[6] The hoard consisted of gold, silver, and precious stones, mainly emeralds, the result of piracy from Spanish ships along the west coast of South America.

[7] Queen Elizabeth II visited the Castle on 25 July 1962 accompanied by the Lord Lieutenant of Cornwall, Sir Edward Bolitho, before driving to Fowey and embarking in the royal yacht Britannia.

[8] In 2012 the castle and grounds were leased from the Duchy of Cornwall by the landscape designers Julian and Isabel Bannerman, who created a garden there.