Nestling on the side of a deep gorge overlooking a bend of the River Tweed, Neidpath Castle is an altered L-plan keep with rounded corners. The keep dates from the 14th century, but was substantially remodelled in the 16th century with the alteration of the upper storeys. It was altered again the late 17th century, when a wide stair was inserted and an additional storey created beneath the vault of the hall. A small courtyard, with ranges of buildings, was added in the 16th and 17th centuries.
The original entrance was in the south side. The basement of the main block and wing are both vaulted. Turnpike stairs climb within the walls.
The much altered first floor has a wide stair leading to the vaulted hall on the second foor. The third floor is also vaulted. Turnpike stairs lead to the upper floors, and there is a dark pit prison which can only be reached from a hatch in the floor above.
An earlier castle here belonged to Sir Simon Fraser. He defeated the English at Roslin Moor in 1302 but was later captured and executed by the English. The property passed, by marriage, to the Hays in 1312, who built the existing castle.. Mary, Queen of Scots, stayed at Neidpath in 1563, as did her son, James V1, in 1587.
The Hays were Royalists, and in 1650 Neidpath held out against Cromwell's army longer than any other stronghold south of the Forth. Cannon damaged the castle and the defenders were eventually forced to surrender. The castle was later partly repaired, and in 1686 was sold to the Douglas Duke of Queensberry, but passed in 1810 to the Earl of Wemyss and March. Walter Scott and William Wordsworth both visited the castle in 1803. Neidpath was featured in many films.