If my opponent surrounds a small region and I create a larger region that surrounds it, do I capture my opponents pieces or are they safe?
You capture your opponents pieces.
It looks like you can capture the whole baord by creating a cross shaped entwork that connects to all four sides of the board. Of course, your opponent is a fool, if he lets you get away with that.
I have managed to capture over half the board before, but never the entire thing.