public class StaticFilesHttpHandler : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
        if (context.Request.IsAuthenticated)
        {
            // continue with the request
        }
        else
        {
            throw new HttpResponseException(HttpStatusCode.Unauthorized);
        }
    }

    public bool IsReusable => false;
}

routes.RouteExistingFiles = true;
routes.Add("helpRoute", new Route("folder/*.htm", new StaticFilesRouteHandler ()));

public class StaticFilesRouteHandler : IRouteHandler
{
    public IHttpHandler GetHttpHandler(RequestContext context)
    {
        return new StaticFilesHttpHandler ();
    }
}

<handlers>
  <add name="StaticFileHandler" verb="GET" path="~/help/default.htm" type="StaticFilesHttpHandler "/>
</handlers>