What was built to reinforce a weak point in the defences of the castle?  Answer the above question based on the context below:  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 answer to this question is:
two-storey rounded tower