Niddry Castle is an altered restored mighty 15th century keep, now L-plan, consisting of a main block and wing. It rises to four storeys and an attic, formerly within a parapet, but only the corbelling remained. The parapet had open rounds at all corners, but was obliterated when two storeys were added to the building in the 17th century. A small stair-turret in the re-entrant angle led to the garret storey.
The entrance in the re-entrant angle leads through a lobby to the vaulted basement, which contained a dungeon in the wing, and to a turnpike stair which climbs in the thickness of the walls, to all floors. The hall on the first floor has a very large fireplace.
The castle, known as "Niddry-Seton" to distinguish it from Niddri Marischall was a property of the Setons. One of the family, George, Seton, was killed at the Battle of Flodden in 1513. The Setons supported Mary, Queen of Scots, and she came here after she had escaped from Lochleven Castle in 1568. The following day she rode to Hamilton, but her army was defeated at the Battle of Langside, and she fled to England to be imprisoned for the rest of her life until executed at Fotheringhay in 1587. Niddry was acquired by the Hopes in the 17th century, but abandoned soon afterwards. The castle is currently occupied by an SCA member and is in a liveable condition and is constantly being worked on with great endeavour.
There are the remains of a formerly fine massive walled garden.