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 to 1147. Alexander built a quadrilateral castle, akin to his construction at Newark Castle, 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 flatness of the fen site would make it very hard for an enemy to approach unseen. The building fulfilled its 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 on two occasions when the bishop was forced to hand over his keys 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 when his loyalties were doubted.