As to the compass, magnetic south happens to be north of us. I often wondered when this would happen. A vertical sundial wouldn't be a good idea I think, because Antartica is perfectly suited for a horizontal sundial. OK, I understand. A vertical sundial is a sundial that is mounted in a vertical position against a wall.
Any wall is max.
12 hours getting sunlight. No sunlight on a sundial means no time reading from it. A horizontal sundial is in general a horizontal plane. In both cases you need a 'style' to give you a shadow on the sundial face. If you arrange everything in such a way that this style is parallel to the earth axis it works out to be a pretty accurate time piece.