You cannot even ENTER a hex that has both a Casa and a campesino in it. Since you can't even inter the space, you can't possibly attack the campesino there, even if you come from higher ground or with greater numbers.
FAQ for one Certain Game
A campesino can pass through a space with an opponent's Casa, but he cannot stop there. He can't even pause long enough to build a dam before the people living in the Casa chase him off. So, you CANNOT build a dam with a worker that is on a hex with an opponent's Casa. If you want to build a dam on the edge of a hex with a Casa, you will have to place a worker on one of the neighboring hexes. Yes, this does mean that if you build two Casa next to each other, no other player will ever be able to build a dam between your two "houses."