Geese are deceptively aggressive, loud and are very territorial. Those three qualities make for quite a well-rounded package for a guard animal that should deter most intruders. To prove this, many animal farms and most famously, the Chinese police, have effectively used geese as an alternative to guard dogs. The fact that geese have often been described as “aggressive pieces of shit” should be enough of a reason for you to at least try them out.
As an added consolation, geese can be eaten, lay eggs and are relatively inexpensive to maintain.