You know a good rule of thumb -- if your landlord ever gives you this restriction but doesn't really give you a reason, just go to the police right then and there and save yourself of the embarassment of being the clueless person who looked the other way while something horrible was going on.
As someone who rents rooms in my home, I can say I don’t necessarily think that restriction would be a red flag. It’s not one I’ve ever had, but I’m highly selective about the sort of people I rent to (most are the sort I’d trust with my credit card info, and I occasionally do). If someone is renting rooms to strangers in their own home and can afford to be that selective because they really need the rent money, then it’s not unreasonable to have some part of the house that’s firmly off-limits, where you feel that valuables or items of great sentimental value are safe.
The WIFE, on the other hand, should have figured out that something was seriously amiss, with her husband often spending the entire night in the cellar and forbidding her to go down there even momentarily while he was there.