Sleaford Castle Sleaford Castle is a medieval castle in Sleaford , Lincolnshire , England . Built by the Bishop of Lincoln in the early 1120s it was habitable as late as 1555 but fell into disrepair during the latter half of the 16th - century . Two English monarchs are known to have stayed at the castle , King John and Henry VIII Medieval era . The castle was built between 1123 and 1139 by Alexander de Blois , Bishop of Lincoln from 1123 - 1147 . Alexander built a quadrilateral castle , akin to his construction at Newark , with square towers and massive keep . He sited it on flat fen rather than on high ground , perhaps even replacing an earlier moated manor house on the site . This shows he intended it for a manor house ( for storing the produce from episcopal land , administering episcopal estates and providing accommodation for the bishop and his entourage when he visited the area ) rather than for defence , though the fen site would make it very hard for an enemy to approach unseen . It fulfilled the manor house function for most of its life , never withstanding an armed attack or siege but becoming one of the chief episcopal strongholds and an agricultural focus for the Bishop ’ s estates in Sleaford and elsewhere . An outline of a 40 by 15 metre tithe barn ( said to be the largest in Lincolnshire , and with a cattle shed and hay loft attached ) can still be seen in the southern half of the castle - local inhabitants would pay their tithe ( or rent for farming the land ) to the bishop , either by time working for the bishop ( either on his land or serving in his army or castle garrison ) , a share of their crop , or a cash sum . A dam was placed across the river Slea at the end of Westgate , with a two - wheeled watermill behind it , producing a large pond to provide fresh fish for episcopal celebrations ( an orchard just outside the castle provided fruit , and a dovecote to the east of the barn meat ) along with rushes and thatch for roofing . The nearest the castle came to a siege were two occasions when the bishop was forced to hand over his keys to the king when his loyalties were doutbtful - specifically , to king Stephen during the Anarchy ( by Alexander himself , to buy his release after Stephen ' s successful siege of Newark Castle ) and to Edward II in the 1320s . King John spent a night in the castle in October 1216 just after his disastrous crossing of the Wash and just before his death , and in 1430 Bishop Richard Fleming died here . Early modern period . Henry VIII stayed at Sleaford twice ( once in 1541 with his queen Catherine Howard ) and held a State Council at the Castle . The castle passed into the hands of the Duke of Somerset in 1544 , from whom it was confiscated by the crown in 1546 . On both occasions , and in 1555 , it was still said to be defensible and habitable . John Leland described it at this time as well maintained with a gatehouse , which housed two portcullises , and a high central tower , ' but not sette upon a hille of raised yerth ' . Decline . However , it soon began to fall into disrepair during the second half of the 16th century , starting with the timber and lead roof being taken . These were reused in buildings in the town such as the ' Manor House ' , some of which survive to the present day . The process of decline continued under the ownership of the Carre family . In 1604 it was described as ‘ the late fair castle ’ , suggesting it had been largely or even fully dismantled before 1600 . An early 18th - century engraving of the castle shows a ruin , but with a considerable amount of stonework still visible . Present . The visible remains are now only a moat , a scrap of masonry ( one small , toppled portion of a wall in the north - east corner of the inner bailey ) and associated earthworks . It is now a scheduled monument and a Grade II listed building protected by law . It is also cultivated for wildlife .