What were baileys built from in Medieval Times?

Answer 1

A bailey is strictly an area enclosed by a protective wall and a moat or ditch - from the Anglo-Norman French word bail, baile or baillie, a protected piece of land.

So baileys themselves were not built from anything - it was their defensive walls that were built. These were originally built from the earth excavated from a surrounding ditch, with the earth built up into a high bank. This was then topped with a palisade of wooden posts.

Later, as castles were rebuilt in stone during the 12th century, walls of stone were added, with towers at intervals, and the ditch was widened and deepened to create a moat.