I have 2 male neutered cats. A few times they howled and seemed very upset when they were inside the house, while a strange cat was outside the back door. Is it a territorial thing?

Yeah It is a territorial thing.. I have a lot of strange cats peeking in my windows and my male howls at them lol

Yes,

Cats, in general are territorial, and will definitely hiss, spit or howl If they see one of their own, hanging out near their pad.

Yes. Male cats, even neutered ones, are very territorial. I've had two male, neutered cats who would freak out when another cat came to the door.

It's a very territorial thing, since they can no longer spray to mark their "territory" the next best thing is a vocal warning. I have the same thing happen with my inside/ouside cat, he will not let another cat, male or female onto our property. He makes it very clear this is HIS home.

A definite yes. The neutering takes away the spraying to mark territory in most cases but does not eliminate the need to establish their own territory. I think cats are mostly little lions and will tolerate other cats in the same household or pride but will naturally want to repel others. The howling is a warning. Quite musical if you are tone deaf. All cats are territorial.

Yes this is territorial behaviour. I have 2 cats, one male and one female and both act like this when there is another cat outside. Basically your boys are telling the other cat that 'this is our house, go away!' they fluff up their bodies to make themselves appear bigger and scarier, and the howling is to let the other cat know that they mean business and is basically a warning of 'don't mess with us!'

Absolutely, cats in general are territorial animals - regardless or gender (I have two spayed female cats who rival countless male cats I've had over the years in terms of how territorial they can be), your cats are simply trying to discourage the feline visitors. I would encourage you to attempt to make the area around your home less inviting to neighborhood cats because in some cases territorial disputes like this can end in a peeing contest literally, unfortunately it may mean that your kitties urinate or defecate outside of the litter box which would be bad news for them if you couldn't work with them to get past it. You can help to reduce the chances of your kitties developing behavioral issues like house soiling in response to territorial issues I'd suggest that you consider applying temporary or permanent privacy film to the windows where your kitties would most likely see an unwanted feline intruder - sometimes out of sight really is out of mind for cats.

Male cats usually have a specific territory mainly confined to their homes.They do not like any outsiders.