The truth is that the Sussex Spaniel has a reputation for having a good intelligence, but not considered very bright. This, however, comes from people who may misunderstand some of the behavior of the breed and see only a stubborn dog that may take time to house and obedience train. They forget to notice that the dog has skills in game hunting that make it excel in one environment and do poorly in another one. Intelligence isn't just about learning how to sit on command, it's also about how well the dog functions within its native environment.
In the Sussex Spaniel's native environment of England, the dog is highly prized for its contribution to small game hunting. It has a great joy of the hunt and its smaller frame allows it to search the underbrush much more effectively than larger dogs. The dog's coat is a golden liver color that blends in with the trees at sunset, making it hard to spot for the prey. It is intensely loyal and a very friendly dog. It can even retrieve game that has fallen in waterways and bring it back to the hunter.
All of these traits indicate a very smart dog in its proper environment. What happens is that the dog is then taken out of its environment and placed in an urban setting where it is no longer in its element. It is taken out of a pack of dogs and isolated and this triggers separation anxiety in the breed. What results is a dog that is stubborn and will either soil the house or bark to get the message across that it doesn't like being left alone. This dog needs constant companionship, whether in the form of another dog or a human. The same behavior can be witnessed if the dog is not exercised enough. It will get bored and resort to putting up a fuss.
However, if the dog is put to some use, exercised regularly, and kept outdoors, it can make a very loving addition to your family. It loves children and makes an excellent watchdog. It is not aggressive towards strangers. It loves games where it fetches and retrieves things and can even learn to like dog training that challenges it in these particular areas of expertise.
You can't expect a dog that is naturally intelligent to be obedient all the time. They sometimes decide to make up their own minds, regardless of their training. This is true with large guard dogs as well as this smaller watchdog. The key is to socialize them and teach them to respect your house rules without abusing them or punishing them for behavior that comes natural to them.