The main entrance to the castle is through the East Gate at the top of Steep hill and facing the Market Place and cathedral. This dates from the C11th and would originally have been reached over a drawbridge across a wide ditch.
The circle wall was constructed on a steep earth rampart with a deep ditch and enclosed the inner bailey. Patches of the C11th herringbone masonry can be seen on the west wall by the west gate. This would have lead into open country and was built on the foundations of the Roman Wall. It was blocked up in the C19th and only reopened in the 1990s.
The Lucy Tower is a polygonal shell keep overlooking the lower area to the south west of the castle and was the first keep to be built on top of a Norman Motte. It would originally have had wooden buildings round the inside of the walls. The walls have been lowered but the two entrances are original. One gave access to the wall walk, the other to the inner bailey.
By the early C19th the interior of the keep was derelict and was used as a burial place for prisoners who had either died in prison or who had been hanged. The small stone slabs remain, with the initials and year.
The Observatory Tower is built on a smaller motte to the south and is slightly later than the Lucy Tower, presumably to guard the entrance to the castle at the East Gate. The base is a square keep with two rooms. The quirky turret reached by a steep spiral staircase was added in the C19th by the Prison Governor, John Merryweather who was a keen amateur astronomer.
Cobb Hall was built later and is a small tower at the south east corner of the wall with arrow slits in the walls. It may originally have had an extra story on the top. In the early C19th gallows were erected on the roof and all public hangings took place here.
Access is from a door and stairs from the roof, leading to a large vaulted room. The lower level was reached through a hatch in the floor which now has a very steep ladder dropping down into it. It may have been used as a dungeon or store house for valuables.
Against the wall beyond Cob Hall is the Bath House built in 1814 with a well in front of it was the bath house and laundry for the prison. The statue is the head of George III which was once part of a full length statue.
The Courthouse near the West Gate was built in 1826 as an administrative centre for county business and a court of law. It replaced earlier buildings. The low grass roofed building by the side of it is the Heritage Skills Centre.
The large brick building just inside the East Gate is the Georgian Prison. This now has the shop and cafe. Behind it is the Victorian prison and the Magna Carta vault.
There are more pictures “here.”:http://wasleys.org.uk/eleanor/castles/england/midlands_south/lincoln_castle/index.html