Say you've got a program in multiple classes, each of which is in its own file.
You distribute this program to some malevolent, too-smart-for-his-own-good, pimply teenager, like myself.
This hypothetical person, using whatever means, figures out what methods are being used, then writes and compiles a replacement for one of the class files.
This is actually, very specifically NOT what public and private are for. That's a very common misconception. If anyone has any of your program files, they can muck with them freely regardless of 'public' or 'private', because those things clearly have nothing to do with encryption or network security or anything like that.
"Public" and "private" are to save those pimply teenagers from
themselves. Here's a more realistic example: Let's say you have a SortedList class. That SortedList presumably has some 'InternalSetData(location, newData)' method. But in normal operation, it only calls that as part of a much larger sorting operation...you say "add a new value", it makes sure everything is sorted. If you called that InternalSetData thingy by yourself, it might skip the whole 'sorting the list' thing...and then either you, or whoever is forced to use your class, might call that function and wind up with a list that isn't sorted.
It's very important for contractual programming that you use public and private stuff. You can make a guarantee of "As long as you only call public methods, this list will always stay sorted". Your private methods, there's no guarantee--but since you're the only one in control there, well, when someone calls a public method, you can then call as many private ones as you want, and make sure that list is sorted again once you're done doing your job, before the next time someone can call a public one!
Try to disabuse yourself of the notion that "public" and "private" are for security. They're just there to make your life easier. If they aren't making your life any easier, don't use them--and in a tiny little roguelike, well, they might not have THAT much use at all.