Borgarvirki is a cool volcanic plug in North Iceland that used to be a fortress back in the day. It's like a natural rock tower that stands out from its surroundings, making it a perfect spot for Vikings to defend themselves. You can check out some old ruins and even a freshwater well inside. Plus, there are other neat sites nearby, like Hvítserkur rock formation and great seal-watching beaches. Overall, Borgarvirki is a rad spot with a bit of Viking history and awesome views.