The Sussex Spaniel is a dog that is not easily trained. House training can be difficult and they are prone to separation anxiety and stubbornness. They do have good intelligence and will eventually pick up on their training, but it can take some time and some patience. The innate stubbornness doesn't make them less friendly, however, they can try to manipulate a situation to get what they want. Just be firm and consistent with their training so that they get the idea that you won't accept less from them than their obedience.
House training a Sussex Spaniel can be difficult because it will sometimes soil the house to show its displeasure. If you leave the dog alone too long, it might soil the house because separation anxiety has been triggered in the dog. You can crate-train this dog, but you shouldn't leave them in the crate too long. They do need to be exercised regularly and vigorously to grow up healthy and strong. The breed can make a great outdoors dog in temperate climates.
The dog is very friendly and with proper socialization can make a great house dog. They will bond with their handler and this can be used to help motivate them to obey your commands. The dog does require an owner with some patience, as there will be mistakes and there will be times when the dog tries your patience. It could be due to bad behavior from separation anxiety, from stubbornness, or just because it is bored and wants your attention.
One thing that really helps this breed is plenty of exercise. The more active they are, the happier they are and the more willing to please. This comes from their heritage as game dogs and they will like any obedience training that brings out these skills. This can make the training something to keep them busy while bringing in areas they naturally excel in. They have in the past gone through underbrush to collect game and have dived into water to retrieve a bird. If you understand that this is what the dog is best at, you can develop play games that help them to release pent up energies while making them feel good about themselves.
Some trainers offer "respect training" for dogs that have a mind of their own. The Sussex Spaniel might benefit from such training and will help to develop a mutual respect between the two of you. The dog is not aggressive towards strangers and can make an excellent watchdog. It is somewhat protective of its family though, and in those cases may show some aggression. If you train it, you will find the dog to be a very amiable pet that can serve a vital function in your household.