The ruined remains of Fast Castle (also known as Castle Knowe) are located on a promontory on Fast Castle Head, 4 miles (6.5 km) northwest of St Abbs Head and a similar distance northwest of Coldingham on the Berwickshire coast of the Scottish Borders.
The site was occupied from the Iron Age and, although it is not known when the first castle was built, a fortress here was occupied by the English in 1346. A substantial structure existed when the Patrick Dunbar, son of the Earl of March, recaptured the castle for the Scots in 1410 and it later passed into the hands of the Home family. In 1503, Margaret Tudor stayed for a night in the castle on her way to Edinburgh to marry James IV. In 1515, it was destroyed as a result of the power-struggle between Alexander Home, the Great Chamberlain (d.1516), and John Stuart, the 4th Duke of Albany (1481 - 1536), who had become Regent, but rebuilt in 1521. It was taken by the English during the Rough Wooing campaign of 1547 but recaptured before Mary, Queen of Scots (1542-87) stayed here in 1566, only to be taken once again by the English in 1570. The castle then passed through marriage from the Homes to the Logans of Restalrig, wealthy landowners of Edinburgh and Leith. However, their estates were forfeit in 1609 due to their implication in the Gowrie Conspiracy. The castle, by then in ruins, passed through the families of the Earls of Dunbar, Arnots, back to the Homes and then the Halls.
Fast Castle featured in the novel The Bride of Lammermoor by Sir Walter Scott (1771 - 1832), who referred to it as Wolf's Crag.