Suppose we walled off a 1000 km square. These walls would be made of unobtanium (so they don't break), and have no cracks or holes that go all the way through. They are also around 15 kilometres high, to stop birds flying over. No living thing can cross the walls.
This is a major element of my setting. This is definitely not a realistic option for enclosing a geographical area, but it has plot reasons.
However, something this big would have major meteorological consequences. Even a measly 2 kilometer high mountain range makes a huge rain shadow. This thing is quite definitely high enough to prevent some (if not all) clouds from crossing over it.
Basically, I have no idea what this would do to the weather of the area. Assume that the wall went up over the course of a year, and that the area enclosed... For simplicity's sake (as in, I don't have to provide a completely made-up map), let's centre this square on Italy, and keep the walls aligned to the compass. Rome to be precise. If the thing is to big to have an actual square, the corners should have right angles, and the distances form the walls that are opposite each other should be 1000 km measured from the middle to the middle. The wall would not be transparent For thermal stuff, let's say it has the characteristics of a hundred meter thick wall of limestone.
My question is, what would this do to the weather inside, and outside the walls? Could people live inside the enclosed area, or would it quickly become a barren wasteland? If it would become a wasteland, what could I do to keep the walls, but avoid this scenario?