Although you're right that the case statement looks a lot nicer (though ```case; when warrior.enemy?;x; when warrior.stairs?;y...``` is really not much worse if you really want a case statement), I'm going to have to join the chorus in vehemently disagreeing that testing the type is in any way idiomatic ruby.

#is_a? is generally considered a code smell in Ruby. It should not matter if the object is of type Enemy, it responding to the message #enemy? is an interface that any object of any type can implement.