As time passed, some of the early motte and bailey castles needed to be upgraded from earth and timber to stone fortifications. It seemed a good idea to build a stone tower on top of the motte.

Bhe castle builders found that it was unsafe to add a full scale square or round keep, because the enormous weight might cause the heaped up earth of the motte to collapse.

To overcome this, a light-weight form of stone fortification, the shell keep was developed. It was realised that because it was on the top of a steep mound, the keep did not need to be thick enough to resist battering rams.

Clifford's Tower, York, North Yorkshire

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