添加通配符路由规则作为最终规则：

routes.MapRoute("Error","{*url}",new { controller = "Error",action = "Http404" });

任何与其他规则不匹配的请求都会路由到错误控制器的Http404操作，您还需要配置它：

public ActionResult Http404(string url) {
    Response.StatusCode = 404;
    ViewData["url"] = url;
    return View();
}