Complex sentence: William was probably responsible for the construction of new stone curtain walls around the inner bailey, destroying the former earthwork defences in the process, and building a hall and service buildings inside the castle, again in stone.
Split into simpler sentences: William was probably responsible for the construction of new stone curtain walls around the inner bailey, destroying the former earthwork defences in the process. The inner bailey was levelled and William built a hall and service buildings inside the castle, again in stone.