The genus loci (or genius loci) of a place is the feel, the essence, really the personality of a place. It used to, properly, refer to the actual guardian spirit of a place but that usage isn't as common anymore. Whether or not the guardian spirits like this change in usage is a matter for someone else. :)
Every place has a certain feel to it. I'm not sure if everyone notices it or not, but I've found it to be true. You can walk into a place and get an immediate impression of what goes on there and what has gone on there in the past. I'm not just talking about the external things - the lights, the signs, the decor (though all of that does give it's own impression too) - but something must deeper and more basic than that. It's a *feeling*. You can walk into a place and feel welcome despite it being an empty shell of a room. Or you can walk into a room that looks like it should feel welcoming and feel uncomfortable.
For example: I have never felt uncomfortable in our current house. Even when we just moved in and all the noises, etc. were new and there should have been moments of unfamiliarity, there were none. I'm so aware of my rooms (to a lesser extent the rest of the house) and the feeling there that I know without question when someone has been in them, let alone touched something or added or moved. It's actually become a game in my family. My parents or sister will bring something into the house, or go into my room and move an item just *slightly* and wait to see if I know. We've lived here for five years and they've yet to get anything by me. I just *know*, because I know what the place should feel like and movement disrupts the feeling. *shrug*
However, in our last house there was always the feeling of someone moving around in the house, even when no one was there. It was unsettled. Not unpleasant or unwelcoming, but restless. And to a further extreme, there was one church that I went to quite often the year we lived in Wisconsin. I'm not sure *why* - I think maybe it was an after school program or something, but I don't remember exactly. I do remember that it was a cute old church - very pleasant looking. And I was *never* comfortable there. I spent the entire time I was there, every time, with my back to a wall because there was a distinct feeling of 'go away, you're not welcome'. I never discussed it with the other kids but I assume that they all felt the same since we all hi-tailed it out of there as soon as we could every time (we'd even rather go play in the graveyard adjacent than hang out in the brightly lit rec rooms with their cushy couches, tvs, and games). So, see? The look of the place didn't match the feel.
Place can be...I guess the word I want is infected by the things that happen there. There's a reason that houses where murders or suicides have taken place feel different than those where none has. And I know, I know, some people think 'well, it's only because you *know* about it.' And I'm sure that some of the time that does have an effect. But not always. People have long reported walking into a place and feeling the wrongness of it only to find out later about the bloody death that occurred.
I sometimes wonder if it's the genus loci of the place that causes the bloodshed in some cases. For instance I read recently of a place where three murders took place. Each about 20 years apart. Not only in the same building (which had different functions in each instance) but in the same corner of the same room. Is there something there that causes people to be more prone to certain kinds of violence? All three of the murders were done with sharp objects. The first was a knife and the other two were both broken bottles. Or was the land okay until the first murder and then that stain was so profound that it twisted the genus loci and subtly encouraged the other later attacks?