Cragside is a Tudor Revival country house near the town of Rothbury in Northumberland, England. Dating to the Victorian era, it was the home of William Armstrong, 1st Baron Armstrong, founder of the Armstrong Whitworth armaments firm. An industrial magnate, scientist, philanthropist and inventor of the hydraulic crane and the Armstrong gun, Armstrong also displayed his inventiveness in the domestic sphere, making Cragside the first house in the world to be lit using hydroelectric power. The estate was technologically advanced: the architect of the house, Richard Norman Shaw, wrote that it was equipped with "wonderful hydraulic machines that do all sorts of things". In the grounds, Armstrong built dams and lakes to power a sawmill, a water-powered laundry, early versions of a dishwasher and a dumb waiter, a hydraulic lift and a hydroelectric rotisserie. In 1887, Armstrong was raised to the peerage, the first engineer or scientist to be ennobled, and became Baron Armstrong of Cragside.

The original building consisted of a small shooting lodge which Armstrong built between 1862 and 1864. In 1869, he employed the architect Richard Norman Shaw to enlarge the site, and in two phases of work between 1869 and 1882, they transformed the house into a northern Neuschwanstein. The result was described by the architect and writer Harry Stuart Goodhart-Rendel as "one of the most dramatic compositions in all architecture". Armstrong filled the house with a significant art collection; he and his wife were patrons of many 19th-century British artists. Cragside became an integral part of Armstrong's commercial operations: honoured guests under Armstrong's roof, including the Shah of Persia, the King of Siam and two future Prime Ministers of Japan, were also customers for his commercial undertakings.

Following Armstrong's death in 1900, his heirs struggled to maintain the house and estate. In 1910, the best of Armstrong's art collection was sold off, and by the 1970s, in an attempt to meet inheritance tax, plans were submitted for large-scale residential development of the estate. In 1971 the National Trust asked the architectural historian Mark Girouard to compile a gazetteer of the most important Victorian houses in Britain which the Trust should seek to save should they ever be sold. Girouard placed Cragside at the top of the list; in 1977, the house was acquired by the Trust with the aid of a grant from the National Land Fund. A Grade I listed building since 1953, Cragside has been open to the public since 1979.

William Armstrong was born on 26 November 1810 in Newcastle upon Tyne, the son of a corn merchant. Trained as a solicitor, he moved to London before he was twenty. Returning to Newcastle, in 1835 he met and married Margaret Ramshaw, the daughter of a builder. A keen amateur scientist, Armstrong began to conduct experiments in both hydraulics and electricity. In 1847, he abandoned the law for manufacturing and established W. G. Armstrong and Company at a site at Elswick, outside Newcastle. By the 1850s, with his design for the Armstrong Gun, Armstrong laid the foundations for an armaments firm that would, before the end of the century, see Krupp as its only world rival. He established himself as a figure of national standing: his work supplying artillery to the British Army was seen as an important response to the failures of Britain's forces during the Crimean War. In 1859, he was knighted and made Engineer of Rifled Ordnance, becoming the principal supplier of armaments to both the Army and the Royal Navy.

Armstrong had spent much of his childhood at Rothbury, escaping from industrial Newcastle for the benefit of his often poor health. He returned to the area in 1862, not having taken a holiday for over fifteen years. On a walk with friends, Armstrong was struck by the attractiveness of the site for a house. Returning to Newcastle, he bought a small parcel of land and decided to build a modest house on the side of a moorland crag. He intended a house of eight or ten rooms and a stable for a pair of horses. The house was completed in the mid-1860s by an unknown architect: a two-storey shooting box of little architectural distinction, it was nevertheless constructed and furnished to a high standard.

Armstrong's architect for Cragside's expansion was the Scot R. Norman Shaw. Shaw had begun his career in the office of William Burn and had later studied under Anthony Salvin and George Edmund Street. Salvin had taught him the mastery of internal planning which was essential for the design of the large and highly variegated houses which the Victorian wealthy craved. Salvin and Street had taught him to understand the Gothic Revival. At only 24, he won the RIBA Gold Medal and Travelling Studentship. The connection between Armstrong and Shaw was made when Armstrong purchased a picture, Prince Hal taking the crown from his father's bedside by John Callcott Horsley, which proved too large to fit into his town house in Jesmond, Newcastle. Horsley was a friend of both, and recommended that Shaw design an extension to the banqueting hall Armstrong had previously built in the grounds. When this was completed in 1869, Shaw was asked for enlargements and improvements to the shooting lodge Armstrong had had built at Rothbury four years earlier. This was the genesis of the transformation of the house between 1869 and 1884. Over the next thirty years, Cragside became the centre of Armstrong's world; reminiscing years later, in his old age, he remarked, "had there been no Cragside, I shouldn't be talking to you today – for it has been my very life".

The architectural historian Andrew Saint records that Shaw sketched out the whole design for the "future fairy palace" in a single afternoon, while Armstrong and his guests were out on a shooting party. After this rapid initial design, Shaw worked on building the house for over 20 years. The long building period, and Armstrong's piecemeal, and changeable, approach to the development of the house, and his desire to retain the original shooting lodge at its core, occasionally led to tensions between client and architect, and to a building that lacks an overall unity. Armstrong changed the purpose of several rooms as his interests developed, and the German architectural historian Hermann Muthesius, writing just after Armstrong's death in 1900, noted that "the house did not find the unqualified favour with Shaw's followers that his previous works had done, nor did it entirely satisfy (Shaw)". Nevertheless, Shaw's abilities, as an architect and as a manager of difficult clients, ensured that Cragside was composed "with memorable force".

As well as being Armstrong's home, Cragside acted as an enormous display case for his ever-expanding art collection. The best of his pictures were hung in the drawing room, but Shaw also converted the museum into a top-lit picture gallery. Pride of place was given to John Everett Millais's Chill October, bought by Armstrong at the Samuel Mendel sale at Christie's in 1875. Armstrong also bought Millais' Jephthah's Daughter at the Mendel sale. Both were sold in the 1910 sale; Chill October is now in the private collection of Andrew Lloyd Webber, and Jephthah's Daughter is held by the National Museum Cardiff.