That's what I love about places in Europe. You go for a hike and stand a good chance of coming upon the ruins of an old castle. Here, where I live, I see a lot rusted cars and the occasional concrete foundation buried under tree roots and moss.
Of course they can. Military objects like this were given up when they became obsolete, because they are hard and uncomfortable to reach. But today most castles are in better conditions as they've ever been and they are protected by law. (what means that the owner has to keep it in original and good conditions).