Horshams only listed ancient monument is the remains of early motte & bailley known as Chennels Brook. There is some evidence that the dry ditch (motte) was changed to water-filled moat by diverting a stream. It was abandoned and slighted sometime after 1150. It was never upgraded from wood and stone. The defence was replaced c1280 by house 200m south, later modified now known as Chennels Brook Farm.
A directory of listed buildings is also on this blog.