If the universe is a donut, that means there could be an outer side and an inner side? Which one are we in?
Think about PacMan, or the old Asteroids game, where going off one end of the screen would put you on the other side. That's a donut. (The 4 corners of the screen make up the single hole*.) Which "side" is the inside? The question doesn't make sense.
*Edit: as rightly pointed out below, the location of the hole is an arbitrary choice that comes from trying to map the space to a sphere, and does not actually exist anywhere in the space itself.
An interesting experiment is this: imagine yourself existing in the space, which is otherwise empty. PacMan alone in the middle of the screen. Throw a stretchy rope to yourself, horizontally or vertically, catch the other end, and tie it together. Then walk around the space without turning the rope at all. Notice that no matter how you walk around, the rope will always be the same length. Now imagine the same thing on the surface of a sphere. Walking around makes the rope larger or smaller, and there's always a point you can walk to where the rope will completely collapse to a single point.
The edges are more interesting, two of them go around the 'hole' of the 'donut' and the other two wrap 'around' the 'donut' itself (i.e. around the dough if it was an actual american style donut). There's no way to tell which is which.
These edges have the interesting property that you can't shrink them to a point (compared to say a loop on a globe which you can make smaller until its a single point). Except when the donut is not hollow in that case one of the loops becomes contractible, turning the space into the equivalent of a circle.
The discontinuity shows up when you try to continuously map the space to the surface of a sphere. You can almost do it, except for one point. Different nearly-continuous maps have a different point of discontinuity -- it's basically your choice when doing the mapping. I think the 4 corners feels like a natural place for the discontinuity when I visualize that mapping -- and scrolling feels like selecting a different mapping -- but indeed it could be any point in the space if you visualize that mapping differently.
If you were in a two-dimensional universe, you wouldn't be on the paper, you would be a patch of the paper.
Edit: Just looked it up. We're both right, sort of. There were warp tunnels on the sides, but not the top and bottom. PacMan was a semi-torus. IOW a cylinder.