The following text is from the original listed building designation:
WEST MALLING
TQ6757 ST LEONARDS STREET 1156-0/2/128 St Leonard's Tower
GV I
Former castle, Built by Gundulf Bishop of Rochester 1077-1108 as his fortified residence or castle. It is one of the best examples extant of an Early Norman Keep. Built of stone with tufa dressings, 32ft square at the base, 60ft high on its north and east faces and 70 ft on the 2 other faces, the difference due to the inclined surface of the rocky shelf on which it is built. Corner buttresses. 3 stages. The 1st stage has traces of herringbone-work. Central buttress to left side elevation. 2nd stage has 5 round-headed arches on the east side, only the central one open, the rest blank. 2nd stage has 2 round-headed arches to east side and one to south side. West side has round-headed entrance to ground floor, round-headed arch to first and second floors and loop lights to buttresses. North side has 1 round-headed window. Internal evidence to suggest there were upper and lower floors, the lower only about 5ft above ground level. (Buildings of England: Newman J: West Kent and the Weald: 580).
Listing NGR: TQ6758657082 (1)

Description from record TQ 65 NE 3 :
[TQ 67595708] St. Leonard's Chapel [NR] (Remains of) (NAT) (2) [TQ 67595708] St. Leonard's Tower [NR] (3) [TQ 67595708] St. Leonard's Church [NR] (Remains of) (NAT) (4) St. Leonard's Tower, although frequently called a keep, is the west tower of a chapel, some remains of which still stand. Built before 1115, it is 32 ft. square, of ragstone and tufa dressing and clasping buttresses, and is divided into two stages. In 1769 it was 71 ft. high but the roof and upper part have been dismantled and it is now about 60 ft. high. Of the chapel, which once measured 70' by 30' all that remains is the North wall running from the Tower towards the road. (5) St. Leonard's Chapel, a scheduled monument, is correctly described above. The north wall of the chapel can be traced for a distance of approx. 21.0m NE from the Tower. It forms the lower courses of the garden wall of Malling Place. A 25" survey carried out. See GPs AO/59/55/4 from east and 55/5 from west. (6) Checked and correct. (7) St. Leonards Tower. A small Norman Tower, remarkably preserved, perched on a sloping shelf of rock. The architecture points to a date c1100. In 1198 St Leonard's cemetery is mentioned, and there are later references to a chapel. Yet this is no church tower, but a freestanding keep tower. Full architectural description.(8) St. Leonards. West Malling. One of many stong towers attached to churches for refuge in case of attack. (9) St Leonard's Tower. This has been described by Mr. H.J. Parker as the oldest Norman keep in England; but despite it castellated appearance, Mr Livett showed that it was the tower of a destroyed church of St Leonard given to Malling Abbey by Bishop Gundulf, whose peculiar style of building it is well exemplified.(10) St Leonard's Tower. Grade I. Ancient Monument. This is all that remains of the once fortified residence or castle of Gundulph, Bishop of Rochester, who founded the abbey in West Malling. It is one of the best examples extant of an early Norman Keep.(11) [For full description see list]. Additional bibliography. (12)

Archive material (13)

<1> English Heritage, List of Buildings of Special Architectural or Historic Interest (Map). SKE16160.

<2> OS 6" 1897 (OS Card Reference). SKE48312.

<3> OS 6" 1909 (OS Card Reference). SKE48324.

<4> OS 6" 1962 (OS Card Reference). SKE48371.

<5> Castles of Gt Britain 1953 263 (ST O4)r (OS Card Reference). SKE38642.

<6> A Short History of W Malling 1951 9 (A Cronk) (OS Card Reference). SKE32914.

<7> Md Milit Arch in Eng 2 1884 9 (G Clark) (OS Card Reference). SKE46670.

<8> Arch Cant 29 1911 245 (A Hussey) (OS Card Reference). SKE34952.

<9> F1 FGA 17-FEB-64 (OS Card Reference). SKE42932.

<10> F2 AC 31-JUL-59 (OS Card Reference). SKE43065.

<11> F3 FGA 17-FEB-64 (OS Card Reference). SKE43395.

<12> The Buildings of England West Kent and the Weald 1980 605 illus (J Newman) (OS Card Reference). SKE50246.

<13> Historic England archive material associated with St Leonards Tower (Archive). SKE53832.

<13> PSA 2 31 1919 176 (OS Card Reference). SKE48764.

<14> Arch J 62 1905 187 (OS Card Reference). SKE36667.

<15> MHLG 2227/11/A May 1950 108 (OS Card Reference). SKE47090.

<16> Monasticon Anglicanum 3 1817-30 311 (ed Caley Ellis and Bandinel) (OS Card Reference). SKE47324.

<17> Arch J 143 1986 316 (M W Thompson) (OS Card Reference). SKE36584.

<18> Field report for monument TQ 65 NE 3 - July, 1959 (Bibliographic reference). SKE3286.

<19> Field report for monument TQ 65 NE 3 - February, 1964 (Bibliographic reference). SKE3287.

<20> GORDON WARD MSS (Collection). SKE6502.