At the beginning of the C20, the south-east parts of the tower, including the gatehouse, were in danger of collapse. For over 700 years the tower's 100,000 tons had been causing the mound to creep.

In 1358 the tower was reported as being split from foundation to summit. In 1596 it was noted that a stone bridge to the keep, which must have provided counterthrust, was no longer extant. In 1684 a fire and explosion split off the south-eastern quadrant of the keep which began to sink forward.

The Prison Act 1877 had transferred ownership of the area from the County Committee to Government, which covenanted in 1880 to maintain "Clifford's Tower which is in the nature of a national monument in such a manner as to prevent the same and every part therof being defaced or injured in its character as a national monument." When in 1902 the area passed back into possession of the County Committee and subsequently the City, the Government's Prison Commissioners had to meet this obligation and reluctantly funded the underpinning and underpropping of the tower.

This was designed by the engineer Basil Mott. It consisted of a concrete platform 6ft deep and up to 17ft wide into which the foundations of the gatehouse and south-eastern walls were consolidated. To support this, five channels 6ft wide were cut into the mound and filled with concrete. This gave in effect five huge flying buttresses extending from the top of the mound to the underlying boulder clay. These [shown in orange] are hidden beneath the turf. As the slippage had been accelerated by water penetration, the mound is internally drained.

The 1935 restoration of the talus of the mound [outside the area shown here] provided further strengthening. It also enhanced the amenities of the Precinct by enabling proximity views of the tower from the foot of the motte. Although the demolition of the prison walls had allowed it to be seen from all directions, green space had begun 20ft in the air, and from the foot of the 1820s revetment the tower had been hidden.