I'm told that slugs can flatten themselves and slither under doors that seem impregnable and have occasionally found them inexplicably in my kitchen from which I have promptly evicted them, with a dose of salt. However, one night I went into my small cellar (was coal hole before my time) and found three sizeable slugs climbing up my freezer. After I'd finished shuddering, I managed to collect them and dispose of them, again with a dose of salt. I suppose there is a way in underground, though I can't move the freezer to find it. I have now sprinkled slug pellets around it.
As for spiders, my niece, a confirmed arachnophobic, went on a course to de-sensitise her and can now actually handle even large hairy spiders!