Complex sentence: Stefan lowered and strengthened the old castle keep to make it an artillery platform and built the half-moon battery which covered the inner bailey adding two small bulwarks or caponiers in the outer moat.
Split into simpler sentences: Stefan lowered and strengthened the old castle keep to make it an artillery platform. New work included the half-moon battery which covered the inner bailey, and two small bulwarks or caponiers in the outer moat.