Im sure this has been touched on before, and its not a big deal. Everynow and then when the game is setup, and two civs are close enough together, one can walk right in and knockout another civ because they sent out all their units to explore. I know this might only happen every so often, but it might be worth looking at where the code is fixed to leave one defender in some fashion. This might also contribute to less civs being knocked out by barbarians when they break out as well.