William the Conqueror built Lincoln Castle, in 1068, on a strategic site that had once been used by the Romans. 166 families who were living on the site had their houses demolished and were forced to provide labour for the construction of the castle. The castle unusually has two mottes, one is topped by a 12th century shell keep called the Lucy Tower, and the other is topped by a 19th century tower that was used as an observatory. A full circuit of curtain walls remain, most of which can be walked on.

The buildings that remain within the castle are from its later use as a prison and court. The Victorian court is still used today, but the prison is no longer in use and can be visited. The prison has the world's only surviving 'Pentonville System' chapel, where prisoners were kept apart in separate cells. Many of the prisoners who were executed at the castle where buried at the base of the Tower mound or inside the Lucy Tower.

The castle also has one of the four remaining original copies of the Magna Carta, which can be viewed in an exhibition area in the old prison building.