Our girl has free access in/out the house via a catflap. For the past couple of months she has been annoyed by a neighbours cat who keeps roaming into 'her territory' ie our garden.
They fight physically which I think results in my cat loosing control of her bladder - or is possible the other cat is spaying on her?? Sounds daft but would you intervene or is it similar to dogs in that they have to establish who is top dog/cat?? The other day they were so vicious that I honestly thought that a small child was being tortured! I also have concerns that the other cat can somehow barge its way in through our sure flap cat flap!
I would go out if I heard a bad fight. I must admit that it was more for moral support than anything else and I doubt my presence was actually needed. My outside-going boys have come home with their shields.