It would completely block line of sight across as it is an obstruction. However, if you and your opponent (or TO) agree that you can see through it then you can do that. However, you would also then need to account for the locations and bases of models within the tower as their volumes could block line of sight.
So, in the end I imagine that it would just be considered to block line of sight like a normal Obstruction.
I would generally advise you to be mindful of the fact that the rules of this game are abstractions. When in doubt, follow the Mystery Science Theater 3000 mantra:
The interpretation that involves the fewest complications is to simply treat the Guard Tower like a solid, LOS-blocking obstacle that is exactly the size it appears to be. Don’t worry too much about how or why the models inside can draw LOS to models outside; they can, and we know the rules for for it, and that is good enough.