Saw this when it came out. I worked in a theater that was showing it. It's fantastic. I highly recommend it. NOT the lousy remake.
I thought the remake was okay, just nothing special. It seems it's one of the few remakes of his work that Carpenter likes. I asked him what he thought about The Thing remake (it's actually a prequel, but it was marketed as a remake) and he said 'No comment', so I assume he was not happy with it.
The blu-ray of Assault is real good. With two separate commentary tracks from both Carpenter and Tommy Lee Wallace (later director of the IT mini-series), who did sound effects and art direction. Wallace worked with Carpenter from Dark Star to Big Trouble in Little China - seems a very relaxed, nice guy just like Carpenter.
Also there are loads of other extras, but there is nothing really about why they chose 13 as the number in the title, why is it Precinct 13?
If you look closely at the exterior of the police station (the exterior was a real police station, as I recall, but not in use anymore), it says: 'Anderson Police Station Division 14'.
I suppose they made it 13, because it's more ominous. And they were unlucky to be there with so few people, when the man came in who was pursued by the street gang. But that's a guess - you can have loads of extras and still a question like that is not answered.
I really love the film. It's one of Carpenter's best and sets the tone of many of his later films. The only minor complaint I would have is that you can tell quite clearly the interior of the police station is on a sound stage. There is no sound at all coming from outside - no traffic, no street noises, no birds. When members of the gang climb through the window later in the film, they are against a background of black, where you would expect to see street lights or trees.
It's a minor complaint, that always slightly bugs me about the movie. But it was a low budget movie of course, and they were still learning about film really when making it. It's also possible it's a conscious choice, to give the police station more of an isolated feel, but I'm not sure about that.