Add following in <System.Web> section in Web.Conf. Put your login page url and default page url where mentioned.    

<authentication mode="Forms">
            <forms cookieless="AutoDetect" defaultUrl="Default Page" loginUrl="Your Login PageUrl"  r timeout="Session Time in Minutes"  slidingExpiration="true"/>
        </authentication>
        <authorization>
            <allow users="*"/>

</authorization>

Now in masterpage load event or in those pages where user authentication is needed do it in Page_Load Event:

if(!Request.IsAuthenticated)//if user is not authenticated
{
   FormsAuthentication.RedirectToLoginPage();//not allow to access and redirect to login
}

    It will work even if you are not using login control. I am successfully using it my project.