Q: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Passage: The current entrance in the north-east occupies the approximate location of the main gatehouse constructed by Gundulf and then rebuilt during 1249–1250. It was pulled down in the 1870s when the enclosure was converted into a municipal garden. An engraving from 1735 by the Buck brothers gives some indication of the gatehouse's form: the gate was an archway between two towers projecting from the curtain wall. It was reached by a stone causeway across a ditch, rather than a drawbridge. A tower containing a postern gate was located in the north-west corner of the enclosure, built at the close of the 14th century to guard the bridge over the Medway. The tower and postern no longer stands, but 19th-century antiquary and engineer G. T. Clark made some notes on the structure while it was still standing and commented that it had mechanisms to lift supplies for the castle from the river. The western part of the stone outer wall, a stretch facing the river, dates from when Gundulf built the first wall enclosing the castle. In the 19th century a revetment was added to strengthen the decaying wall. Like the keep, it was constructed using Kentish Ragstone. This part of Gundulf's wall was 4.5 feet (1.4 m) thick at the base, narrowing to 2 feet (0.61 m) at the top; it rose to a height of around 22 feet (6.7 m). Four embrasures were added to this part of the wall in the 13th century, although the builders imitated Norman design. At the northern end of the 12th-century stretch of western wall are the remains of a building, probably a hall, dating from the 13th century. Although no longer standing, it is known it had a vaulted undercroft.In the south Gundulf's wall survived into the modern period, although has now been dismantled. The current wall is a 19th-century replacement. At the eastern end of this wall, near the southern corner of the castle, is a two-storey rounded tower 30 feet (9.1 m) in diameter dating from the early 13th century. It was built to fill the breach in the curtain wall caused when John's army besieged the castle and to reinforce a weak point in the defences. The section from the tower to the location of the former main gatehouse in the north-east dates from about 1367 to 1370. Two towers were built along the wall, each two storeys high and again using Kentish Ragstone. The one nearest the keep is relatively plain and the northernmost one more elaborate. The latter was intended for use as a residence and in the modern period was converted into a cottage. The wall between these two towers was reduced in the modern period, possibly to give a better view of the cathedral. Apart from the west side, the castle was surrounded by a ditch, much of which has since been filled in.
A:
What was built to reinforce a weak point in the defences of the castle?