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?

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.

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. 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.

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!'

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