What type of stone was replace in the 17th and 18th centrury?
The White Tower is a keep (also known as a donjon), which was often the strongest structure in a medieval castle, and contained lodgings suitable for the lord – in this case, the king or his representative. According to military historian Allen Brown, "The great tower [White Tower] was also, by virtue of its strength, majesty and lordly accommodation, the donjon par excellence". As one of the largest keeps in the Christian world, the White Tower has been described as "the most complete eleventh-century palace in Europe".
The White Tower, not including its projecting corner towers, measures 36 by 32 metres (118 by 105 ft) at the base, and is 27 m (90 ft) high at the southern battlements. The structure was originally three storeys high, comprising a basement floor, an entrance level, and an upper floor. The entrance, as is usual in Norman keeps, was above ground, in this case on the south face, and accessed via a wooden staircase which could be removed in the event of an attack. It was probably during Henry II's reign (1154–1189) that a forebuilding was added to the south side of the tower to provide extra defences to the entrance, but it has not survived. Each floor was divided into three chambers, the largest in the west, a smaller room in the north-east, and the chapel taking up the entrance and upper floors of the south-east. At the western corners of the building are square towers, while to the north-east a round tower houses a spiral staircase. At the south-east corner there is a larger semi-circular projection which accommodates the apse of the chapel.  As the building was intended to be a comfortable residence as well as a stronghold, latrines were built into the walls, and four fireplaces provided warmth.The main building material is Kentish rag-stone, although some local mudstone was also used. Caen stone was imported from northern France to provide details in the Tower's facing, although little of the original material survives as it was replaced with Portland stone in the 17th and 18th centuries. As most of the Tower's windows were enlarged in the 18th century, only two original – albeit restored – examples remain, in the south wall at the gallery level.The tower was terraced into the side of a mound, so the northern side of the basement is partially below ground level. As was typical of most keeps, the bottom floor was an undercroft used for storage. One of the rooms contained a well. Although the layout has remained the same since the tower's construction, the interior of the basement dates mostly from the 18th century when the floor was lowered and the pre-existing timber vaults were replaced with brick counterparts. The basement is lit through small slits.
Caen stone