A fortification has existed on the site from Saxon times, but the current ruin is of Norman origin; a great square stone tower built by Geoffrey de Clinton, Treasurer and Chief Justice of England to Henry I, in about 1125. Henry II took control of the castle during the Revolt of 1173-1174, giving the Clintons another castle in Buckinghamshire by way of compensation.
Work then began to improve the defensive qualities of the castle, continuing during the reign of Henry III and transforming the castle into one of the strongest in the Midlands. The strategic advantages of water defences had long been known, and at Kenilworth a great man made lake was created to defend three sides of the castle. Covering over 100 acres (0.4 km²) it was an expensive endeavour, but the value in keeping siege engines at a distance and as a barrier to assault or mining was immense.
However, after all the work to improve the castle, Henry III granted it in 1244 to Simon de Montfort, Earl of Leicester. Simon de Montfort became a leader in the Second Barons' War (1263-1267) against Henry III, using Kenilworth as the centre of his operations. Prince Edward, Henry's heir, was once briefly imprisoned at Kenilworth before escaping. De Montfort was killed in battle near Evesham on August 13 1265 facing Edward. In 1266, the rebels under the leadership of Henry de Hastings, used the castle as a refuge when Lord Edward surrounded Kenilworth. The Siege of Kenilworth Castle in 1266 is the longest in English history. The extensive water defences provided their intended protection, despite Edward targeting the weaker north wall, defended by only a double moat, employing huge siege towers and even attempting a night attack by barge. The siege was ended on easy terms for the defenders with the Dictum of Kenilworth. The experience gained in water defences at Kenilworth was put to good effect at later castles built in Wales, notably Caerphilly.
Henry III bestowed the castle upon his youngest son Edmund Crouchback. The castle was inherited by Edmund's grandson Henry of Grosmont, 1st Duke of Lancaster, and then passed to the Duke's son-in-law John of Gaunt.
From 1364 John of Gaunt began the castle's conversion from a pure fortress into something more liveable, work that continued with his grandson, Henry V. The castle remained in royal hands until it was given to John Dudley in 1553. Following his execution Elizabeth I gave it to her favourite Robert Dudley, Earl of Leicester in 1563. Thomas Underhill was named Keeper of the Wardrobe at this time. Dudley further transformed the castle by making the north entrance the main entrance to suit the tastes of Elizabeth, adding the Leicester building, a large apartment and a residential block overlooking the lake.
Elizabeth visited Dudley at Kenilworth Castle several times in 1566, 1572, and 1575. The last visit is especially remembered when Elizabeth brought an entourage of several hundred. No expense was spared for the July visit that lasted 19 days and is reputed to have cost Dudley £1000 per day, an amount that almost bankrupted him. Dudley entertained the Queen with pageants, bear baiting and lavish banquets that surpassed anything ever before seen in England.
The festivities are said to have been the inspiration for Shakespeare's A Midsummer Night's Dream. William Shakespeare was just 11 years old at the time and from nearby Stratford-upon-Avon. He could well have been among the crowd of locals that would have gathered to witness the occasion with its expensive and lavish arrangements.
Sir Walter Scott wrote an 1821 novel describing the royal visit, and Sir Arthur Sullivan wrote a choral work in 1864 about it. Elizabeth is said, according to local legend, to have tasted the first potato brought into the country at Kenilworth Castle. Unfortunately she ate it raw, disliked it and threw it out the window where it grew in an area now known as Little Virginia.
The castle returned to the Crown on Dudley's death. In the English Civil War, the castle was stormed and looted by Parliamentarian troops. In common with many English castles, Kenilworth was slighted (rendered indefensible) after the Civil War. One wall of the keep was blown up, and battlements and the great water defences were destroyed, in 1656.
In 1660 Charles II gave the castle to Sir Edward Hyde, whom he created Baron Hyde of Hindon and Earl of Clarendon. The castle remained the property of the Clarendons until 1937 before passing into the possession of John Davenport Siddeley, 1st Baron Kenilworth. The family presented the castle to Kenilworth in 1958 and English Heritage has looked after it since 1984.
In 2005 English Heritage announced that after archaeological investigations revealed more details of the original garden, it will be restoring the garden more closely to its Elizabethan form. A fountain and aviary will be reconstructed. The project is scheduled for completion in 2008  In December 2008 plans were put forward to re-fill the original mere around the castle. As well as re-creating the look of the castle it is hoped that the mere will be part of the ongoing flood alleviation plan for the area and the lake could be used for boating and other waterside recreations.
from the south]]
The constables of Kenilworth Castle include:
- Henry de Hastings, 1st Baron Hastings (1265-1266)
- Philip Marmion (appointed 1267)
- Hugh de Quilly (c. 1310-1320)
- John Deyncourt (c. 1382)
- Ralph Boteler, 1st Baron Sudeley (appointed 1433)
- Thomas Grey, 2nd Marquess of Dorset (1529-1530)
- John Huband (Constable of Robert Dudley, Earl of Leicester)
- Robert Dudley, styled Earl of Warwick (1611-1649)
- Siege of Kenilworth
- Christopher Candy, For the Sake of a Keep: The Siege of Kenilworth, 1267
- English Heritage - visitor information
- English Heritage - information for teachers - includes plans, reconstructions and bibliography
- Photos of Kenilworth Castle today on geograph.org.uk