Why is he acting so territorial?

[smg680]

My family has two cats who have adopted us and come over for food. They are stray outdoor cats, affectionately nicknamed Number 2 and Number 3, and my dad made a little shelter for them for the cold weather. Ambre has never been with another cat in our house. He's been an indoor cat since he was AWOL for thee days.

A few weeks ago, he saw a strange cat, and he went NUTS! My dad was holding him and trying to calm him down (big mistake I think), but he bit and scratched him as he tried to get out to confront the intruder. Dad's doing okay now after a trip to the doctor who gave him some medicine (he had a tetanus shot earlier, so that's no problem, and Ambre is up to date on his shots, including rabies).

Why is Ambre acting like this? He even chased his own brother out a few years ago on Father's Day (shame on you treating your own brother like that!) and we're all hoping Creamy has since found a nice home since he doesn't come by anymore. Why is Ambre being so territorial?