The fantastic ‘Hampshire Treasures’ resource gives the following info – “Malwood Castle. Earthworks can still be traced, though overgrown by trees and shrubs. House in centre privately owned. Scheduled Ancient Monument no 270.” A Beacon topped the fort site.
The historic (but not ancient) Rufus Stone is not far to the north west at SU270124. This is a standing slab which commemorates the legendary site of the death of William Rufus in 1100. The stone was erected by Earl De La Ware in 1745.