See how the Castle has evolved throughout history with these key historical milestones:

The Castle at the time of Hotspur’s birth in the 1360s’

In 1309, Henry Percy, great-great grandfather of Hotspur, purchased a typical Norman-style castle of motte and bailey form. In the following 40 years he and his son converted it into a mighty border fortress. They added towers and guerites around its curtain walls with a strong gatehouse at the entrance and a concealed postern gate to the rear. The gateway to the keep was strengthened with the addition of two massive octagonal towers.
Stone figures were added to the tops of the battlements, as was fashionable at that time, either for ornament, or to confuse attackers. This was a medieval device that the 1st Duchess was to copy to excess in the more fanciful mid-18th century castle restoration.

The Castle in 1475

You are now standing in the Barbican, one of the finest surviving structures of its type in Britain. It played a major role in securing the defence of Alnwick Castle. There is an ongoing debate as to the exact date of its construction, with some architectural historians citing an early 14th century date. This time frame is also linked to the major building works then undertaken by the first Lord Percy of Alnwick and his son. Archival evidence, however, shows that building work was carried out in the late 15th century, and that the 4th Earl’s badge was placed over the entrance in 1475. 
The 4th Earl had alterations and repairs carried out to adapt the Barbican to the latest tactics of warfare. The town’s defences were also being strengthened during this period, following devastating raids by the Scots in the first half of the 15th century.
The purpose of the Barbican
The Barbican had numerous purposes. It stood as the first line of defence at the castle’s most vulnerable spot. Up until the 18th century this was the only entrance to the castle, apart from the concealed postern gate. For counter-attack, it was possible within its high walls, safe from enemy view, for castle forces to mass in order to spring an assault on besiegers.
The Barbican controlled everyday access to the castle by funnelling approaching traffic on foot, horseback or in a wagon, onto the causeway within its walls. Once the outer door was closed, the castle porter and Gatehouse guards could check all visitors. Without this system, the Gatehouse stood, in times of continual border disruption, an easy target for even a small determined force of raiders. 

Early in the 16th century, the castle was assessed and written off in defensive terms as not liable to abide the force of any shot or to hold out any time if it should be assaulted. In 1567, the 7th Earl employed George Clarkson to survey the castle and his northern estates. His detailed account, together with the plans drawn by Treswell in 1608, enable us to be quite accurate about how the castle looked and what the buildings were used for during this period.
Clarkson also describes the condition of the buildings, mentioning that the Ravine Tower was "so rente that it is mooche like to fall", as indeed it did later in the 17th century. Clarkson describes the corner tower in the inner bailey as having no back to it, being two storeys and only as high as the battlements, and being used for storing hay.
By 1608, Treswell shows it as three storeys high with a stone back. Perhaps this reflects the creation of the Record Tower between 1567 and 1608.

During the 17th century, the castle fell into disrepair, both through neglect because the Percy family was mainly resident in the south, and through damages done in wartime. An eye witness account records the damage done to the castle during the Civil War:
I cannot sufficiently relate the Augmentation of the spoyle of his Lordship’s howse, eaven by both parties, burning wood, takeing away all the Iron barrs, bolts and Lockes of doores and doore bands to great dammage of the howse, distroying of meadowes soe as I knowe not where to make any provisions of hay for your use, nor dare I adventure to repare or put any thing in good order by reason of badnes of tymes and the incertainyty of amendment (the lead of the house is yet well saved and that is all).
The best visual evidence for the appearance of the castle at this date is a painting by Peter Hartover (fl.1674-1690), which depicts the ruinous chapel and trees growing out of the stonework.

Transforming the castle from a decaying garrison fortress to a palace was conceived in a high gothick style to the designs of the architects Daniel Garrett, James Paine and Robert Adam in the 1760s. Work on the parklands carried on in tandem. Weirs were built on the River Aln to slow the water flow with the effect of enhancing the landscape and providing a reflective surface for the newly restored castle.
The terrace where you now stand was created to view and admire the transforming scenery. The 1st Duchess visited Alnwick each summer, particularly enjoying her carriage rides throughout the surrounding countryside. In one of her notebooks she lists and names no fewer than 98 different rides that could be made from the castle.

The 4th Duke disliked the ‘fairytale gothick’ style and inconvenience of the castle created by the restoration undertaken a century previously. He employed the architect Anthony Salvin to restore a more authentic medieval border fortress appearance to the exterior. For the state apartments, however, Algernon chose the lavish style of an Italianate palazzo. Improvements were made across the castle site exploiting new technologies of the Victorian age.

By the time Hugh Percy entered the dukedom in 1940, the large team of live-in domestic servants he had known as a boy was no longer in existence. This left vast areas of the castle unused and unoccupied. These provided facilities, first for the accommodation of Alnwick Teacher Training College, and then, from 1981, for St Cloud State University students from Minnesota in the United States.

The Duke and his family share their home with Estates Office staff, American students from St Cloud State University residential programme and the general public. Recent years have witnessed an extensive programme of conservation, repair and refurbishment to the fabric of the building, both exterior and interior. Roof leads have been replaced; essential masonry repair and re-pointing has been undertaken, as well as conservation work and refurbishment of the interiors. Such works both preserve the castle and continue its development.