Grimsthorpe Castle is a country house in Lincolnshire, England 4 miles (6.4 km) north-west of Bourne on the A151. It lies within a 3,000 acre (12 km²) park of rolling pastures, lakes, and woodland landscaped by Capability Brown. While Grimsthorpe is not a castle in the strict sense of the word, its character is massive and martial – the towers and outlying pavilions recalling the bastions of a great fortress in classical dress. Grimsthorpe has been the home of the de Eresby family since 1516. The present owner is Jane Heathcote-Drummond-Willoughby, 28th Baroness Willoughby de Eresby, granddaughter of Nancy Astor, who died at Grimsthorpe in 1964.
The building was originally a small castle on the crest of a ridge on the road inland from the Lincolnshire fen edge towards the Great North Road. It is said to have been begun by Gilbert de Gant, Earl of Lincoln in the early 13th century. However, he was the first and last in this creation of the Earldom of Lincoln and he died in 1156. Gilbert's heyday was the peak time of castle building in England, during the Anarchy. It is quite possible that the castle was built around 1140. However, the tower at the south-east corner of the present building is usually said to have been part of the original castle and it is known as King John's Tower. The naming of King John's tower seems to have led to a misattribution of the castle's origin to his time.
Gilbert de Gant spent much of his life in the power of the Earl of Chester and Grimsthorpe is likely to have fallen into his hands in 1156 when Gilbert died, though the title 'Earl of Lincoln' reverted to the crown. In the next creation of the earldom, in 1217, it was Ranulph de Blondeville, 4th Earl of Chester (1172–1232) who was ennobled with it. It seems that the title, if not the property was in the hands of King John during his reign; hence perhaps, the name of the tower.