A pretty, young fashion model gets a new apartment in the city for a deal that seems to good to be true. Because this is a horror movie, of course it's too good to be true. She meets an assortment of strange, ingratiating neighbors who throw weird parties that may or may not turn into orgies and do things like spontaneously masturbate in front of her. Before you know it, she's also being visiting by creepy naked dead people in the night. Could the apartment be... the set of a porn movie? Wait, I mean... the gateway to hell?
I think the key is, don't be Catholic. It's always the Catholics who have to battle demons and shit and fight for the salvation of someone's soul. Just be a Jew or Muslim or, heck, join a different sect of Christianity like Protestant, or whatever, and you'll lead a peaceful, devil-free life.
The Sentinel is a particularly silly entry in the whole Exorcist/Rosemary's Baby-ripoff genre they had going on in the 70's. It's good for maybe a few WTF moments (like the aforementioned masturbation scene) and the occasional enjoyable bit of imagery (all the ghoulish things that come out during the climax), but mostly it's goofy and stilted, packed full of awkward dialogue ("someone is staring at me from the 5th floor up there") and bad ideas for scary scenes (I swear one scene tries to mine tension out of whether or not the heroine can set down a wine bottle in the proper direction for a commercial shoot).
I have trouble rating these kinds of movies sometimes, when they have a certain camp appeal that I appreciate. Do I credit it for occasionally amusing me, even though that amusement wasn't intentional? In this case, considering that this was by the same director as Death Wish 3, it would take a lot more for me to praise it on ground of camp.