Though now a ruin, Sanquhar Castle was once the stronghold of a powerful family. Bound on the west side by the River Nith, to the north side by Townfoot Burn, the castle was made even more defensible by a deep ditch running around the remainder of its boundary.

The lands originally belonged to the Edgar family, then passed to the Ross family who built the first castle here. The Crichton family obtained the lands during the reign of Robert the Bruce, and they built Sanquhar Castle in the late 13th century. The name is derived from the Scots Gaelic Seann Cathair, which means "old fort."

The earliest part of the Crichton castle, built around 1360, was a three-story tower overlooking a small courtyard enclosed by a curtain wall. Around 1400, a new block was added to the castle which had cellars on the ground floor and a hall above. Another D-shaped tower was added around 1450.

The Crichtons were made Earls of Dumfries in 1633, and they sold the castle in the mid-17th century. It passed into the hands of Sir William Douglas, the 1st Duke of Queensbury, the man responsible for Drumlanrig Castle near Thornhill. Douglas did not like his new castle and stayed only one night at Drumlanrig before returning to Sanquhar. After his death the family moved to the grander Drumlanrig Castle, and Sanquhar Castle was left to crumble into ruin.

In 1895, the 3rd Marquess of Bute, John Crichton-Stuart, purchased the castle and began an attempt to restore his ancestral home. The squarer, more structurally sound parts of the castle rebuilt at this time are easily identified.  But after the Marquess died in 1900, this restoration was abandoned and the castle began to deteriorate once again.

What is left of the castle now is an interesting mix of the original stonework and the restoration work.  A four-storey tower stands at one corner of the site, while a ruined hall and later wing with a gateway passage and semi-circular tower survive. The castle has had many notable visitors over the years including Mary, Queen of Scots in 1568, James VI in 1617, while both Robert the Bruce and William Wallace were also said to have visited the castle.

Like any good old castle, Sanquhar has its ghostly legend attached.  Two are said to haunt the castle, one the "White Lady" is said to be the ghost of a young, golden-haired woman called Marion of Dalpeddar who is said to have vanished in 1590.  The skeleton of a girl was found during excavations in 1875 and added a ring of truth to the story of Marion's disappearance.  The other ghost is that of John Wilson, wrongly hanged by the Crichtons, he is said to haunt the place with moans and a rattling of chains.