Norman rulers put up many castles across England in the years right after the conquest. What were those first castles mostly made of?

Explanation

Speed mattered far more than strength at first, and a mound with a wooden tower on it could be raised in weeks by local labour, which is why so many appeared so quickly across a newly conquered country. Elaborate masonry defences are the tempting picture, because that is what a castle looks like in most illustrations, yet such building belonged to a later stage, once rule was secure and the first earthwork sites were rebuilt in more lasting materials.