Lincoln Castle is bounded by a stone curtain wall, with ditches on all sides except the south. From an early stage, the outer walls which enclose the site were built in stone and they date from before 1115. On the south side the walls are interrupted by two earthen mounds called mottes. One is in the south-east corner, and was probably an original feature of William's the Conqueror's castle, while the other occupies the south-west corner. A square tower, the Observatory Tower, stands on top of the first mound, standing above the outer walls to dominate the city of Lincoln. The second mound is crowned by the 'Lucy Tower', which was probably built in the 12th century and was named after Lucy of Bolingbroke, the Countess of Chester until 1138.