 The hexagonal Great Tower was probably begun in the 1430s and 1440s , possibly on the motte of a previous castle . The tower today has lost not only one of its walls but part of its upper structure , and would originally have been three storeys high with probably additional machicolations on top similar to those on the gatehouse . It was designed to be a self @-@ contained fortification , with its own water and food supplies , and luxurious quarters lit by large windows on the upper floors . Originally the tower was reached by a bascule drawbridge , usually considered to be drawn from contemporary French designs , such as those at Ferté @-@ Milon and Vannes . This drawbridge was designed to have two parts — a wide , heavy bridge that would be raised or lowered when the family was in residence , and a thinner bridge , easier to lift , designed for the use of servants at other times . The Herberts used the bridge as their badge , and it can be seen in the carved window designs around the castle . The drawbridge was replaced with a grander stone bridge in the 1460s , probably at a cost of around £ 900 to £ 1000 . An apron wall with six turrets was also added around the tower at around the same time . 
