I don't mind this discussion at all. Sorry for the late reply, ravagekitteh, but I'm out of town on a vacation and been quite busy.
The Archway is a VERY solid piece once put together. I think you could magnetize it, though, I'm not quite sure why you'd do that. The separate pieces by themselves, aren't very substantial. They, in my opinion, wouldn't make good terrain alone. Then you have the issue about the interior, second-floor platform. It's the thinnest piece and I don't think that it'll magnetize very well. Other than the sturdy corner connections, that floor piece is the part that holds the model together and firm.
All that is to say, I think you'd be better served by biting the bullet and going ahead with gluing the model together. It's a really nice model and you'll get lots of use out of it.
If you do decide to go with magnetizing the separate pieces, I'd prime the model parts first, then drill holes where the magnets would go (make sure they have the correct connecting poles pointed in the proper direction!) and then glue them in so that they are sunk far enough so that they don't show. (This is also the problem with the second floor piece. It isn't think enough to dirll into far enough for the magnets to disappear, and if you just glue them to the edge of it, then your model will be pushed apart from the way it's meant to fit together.)
I hope that helps!